diff --git a/httpcore/_async/connection_pool.py b/httpcore/_async/connection_pool.py
index 214dfc4be..6f64c3d99 100644
--- a/httpcore/_async/connection_pool.py
+++ b/httpcore/_async/connection_pool.py
@@ -7,7 +7,7 @@
 from .._backends.base import SOCKET_OPTION, AsyncNetworkBackend
 from .._exceptions import ConnectionNotAvailable, UnsupportedProtocol
 from .._models import Origin, Request, Response
-from .._synchronization import AsyncEvent, AsyncShieldCancellation, AsyncThreadLock
+from .._synchronization import AsyncEvent, AsyncThreadLock, async_cancel_shield
 from .connection import AsyncHTTPConnection
 from .interfaces import AsyncConnectionInterface, AsyncRequestInterface
 
@@ -299,11 +299,16 @@ def _assign_requests_to_connections(self) -> List[AsyncConnectionInterface]:
         return closing_connections
 
     async def _close_connections(self, closing: List[AsyncConnectionInterface]) -> None:
+        if not closing:
+            return
+
         # Close connections which have been removed from the pool.
-        with AsyncShieldCancellation():
+        async def close() -> None:
             for connection in closing:
                 await connection.aclose()
 
+        await async_cancel_shield(close)
+
     async def aclose(self) -> None:
         # Explicitly close the connection pool.
         # Clears all existing requests and connections.
@@ -369,9 +374,9 @@ async def __aiter__(self) -> AsyncIterator[bytes]:
     async def aclose(self) -> None:
         if not self._closed:
             self._closed = True
-            with AsyncShieldCancellation():
-                if hasattr(self._stream, "aclose"):
-                    await self._stream.aclose()
+
+            if hasattr(self._stream, "aclose"):
+                await async_cancel_shield(self._stream.aclose)
 
             with self._pool._optional_thread_lock:
                 self._pool._requests.remove(self._pool_request)
diff --git a/httpcore/_async/http11.py b/httpcore/_async/http11.py
index 0493a923d..569e3cd32 100644
--- a/httpcore/_async/http11.py
+++ b/httpcore/_async/http11.py
@@ -25,7 +25,7 @@
     map_exceptions,
 )
 from .._models import Origin, Request, Response
-from .._synchronization import AsyncLock, AsyncShieldCancellation
+from .._synchronization import AsyncLock, async_cancel_shield
 from .._trace import Trace
 from .interfaces import AsyncConnectionInterface
 
@@ -137,9 +137,8 @@ async def handle_async_request(self, request: Request) -> Response:
                 },
             )
         except BaseException as exc:
-            with AsyncShieldCancellation():
-                async with Trace("response_closed", logger, request) as trace:
-                    await self._response_closed()
+            async with Trace("response_closed", logger, request) as trace:
+                await async_cancel_shield(self._response_closed)
             raise exc
 
     # Sending the request...
@@ -344,8 +343,7 @@ async def __aiter__(self) -> AsyncIterator[bytes]:
             # If we get an exception while streaming the response,
             # we want to close the response (and possibly the connection)
             # before raising that exception.
-            with AsyncShieldCancellation():
-                await self.aclose()
+            await async_cancel_shield(self.aclose)
             raise exc
 
     async def aclose(self) -> None:
diff --git a/httpcore/_async/http2.py b/httpcore/_async/http2.py
index c201ee4cb..da47fbb2d 100644
--- a/httpcore/_async/http2.py
+++ b/httpcore/_async/http2.py
@@ -17,7 +17,7 @@
     RemoteProtocolError,
 )
 from .._models import Origin, Request, Response
-from .._synchronization import AsyncLock, AsyncSemaphore, AsyncShieldCancellation
+from .._synchronization import AsyncLock, AsyncSemaphore, async_cancel_shield
 from .._trace import Trace
 from .interfaces import AsyncConnectionInterface
 
@@ -108,8 +108,7 @@ async def handle_async_request(self, request: Request) -> Response:
                     async with Trace("send_connection_init", logger, request, kwargs):
                         await self._send_connection_init(**kwargs)
                 except BaseException as exc:
-                    with AsyncShieldCancellation():
-                        await self.aclose()
+                    await async_cancel_shield(self.aclose)
                     raise exc
 
                 self._sent_connection_init = True
@@ -160,11 +159,12 @@ async def handle_async_request(self, request: Request) -> Response:
                     "stream_id": stream_id,
                 },
             )
-        except BaseException as exc:  # noqa: PIE786
-            with AsyncShieldCancellation():
-                kwargs = {"stream_id": stream_id}
-                async with Trace("response_closed", logger, request, kwargs):
-                    await self._response_closed(stream_id=stream_id)
+        except BaseException as exc:
+            kwargs = {"stream_id": stream_id}
+            async with Trace("response_closed", logger, request, kwargs):
+                await async_cancel_shield(
+                    lambda: self._response_closed(stream_id=stream_id)
+                )
 
             if isinstance(exc, h2.exceptions.ProtocolError):
                 # One case where h2 can raise a protocol error is when a
@@ -577,8 +577,7 @@ async def __aiter__(self) -> typing.AsyncIterator[bytes]:
             # If we get an exception while streaming the response,
             # we want to close the response (and possibly the connection)
             # before raising that exception.
-            with AsyncShieldCancellation():
-                await self.aclose()
+            await async_cancel_shield(self.aclose)
             raise exc
 
     async def aclose(self) -> None:
diff --git a/httpcore/_backends/auto.py b/httpcore/_backends/auto.py
index 3ac05f4da..9b362764d 100644
--- a/httpcore/_backends/auto.py
+++ b/httpcore/_backends/auto.py
@@ -1,14 +1,14 @@
 import typing
 from typing import Optional
 
-from .._synchronization import current_async_library
+from .._synchronization import current_async_backend
 from .base import SOCKET_OPTION, AsyncNetworkBackend, AsyncNetworkStream
 
 
 class AutoBackend(AsyncNetworkBackend):
     async def _init_backend(self) -> None:
         if not (hasattr(self, "_backend")):
-            backend = current_async_library()
+            backend = current_async_backend()
             if backend == "trio":
                 from .trio import TrioBackend
 
diff --git a/httpcore/_sync/connection_pool.py b/httpcore/_sync/connection_pool.py
index 01bec59e8..fc8c4d7b1 100644
--- a/httpcore/_sync/connection_pool.py
+++ b/httpcore/_sync/connection_pool.py
@@ -7,7 +7,7 @@
 from .._backends.base import SOCKET_OPTION, NetworkBackend
 from .._exceptions import ConnectionNotAvailable, UnsupportedProtocol
 from .._models import Origin, Request, Response
-from .._synchronization import Event, ShieldCancellation, ThreadLock
+from .._synchronization import Event, ThreadLock, sync_cancel_shield
 from .connection import HTTPConnection
 from .interfaces import ConnectionInterface, RequestInterface
 
@@ -299,11 +299,16 @@ def _assign_requests_to_connections(self) -> List[ConnectionInterface]:
         return closing_connections
 
     def _close_connections(self, closing: List[ConnectionInterface]) -> None:
+        if not closing:
+            return
+
         # Close connections which have been removed from the pool.
-        with ShieldCancellation():
+        def close() -> None:
             for connection in closing:
                 connection.close()
 
+        sync_cancel_shield(close)
+
     def close(self) -> None:
         # Explicitly close the connection pool.
         # Clears all existing requests and connections.
@@ -369,9 +374,9 @@ def __iter__(self) -> Iterator[bytes]:
     def close(self) -> None:
         if not self._closed:
             self._closed = True
-            with ShieldCancellation():
-                if hasattr(self._stream, "close"):
-                    self._stream.close()
+
+            if hasattr(self._stream, "close"):
+                sync_cancel_shield(self._stream.close)
 
             with self._pool._optional_thread_lock:
                 self._pool._requests.remove(self._pool_request)
diff --git a/httpcore/_sync/http11.py b/httpcore/_sync/http11.py
index a74ff8e80..12b0eca4b 100644
--- a/httpcore/_sync/http11.py
+++ b/httpcore/_sync/http11.py
@@ -25,7 +25,7 @@
     map_exceptions,
 )
 from .._models import Origin, Request, Response
-from .._synchronization import Lock, ShieldCancellation
+from .._synchronization import Lock, sync_cancel_shield
 from .._trace import Trace
 from .interfaces import ConnectionInterface
 
@@ -137,9 +137,8 @@ def handle_request(self, request: Request) -> Response:
                 },
             )
         except BaseException as exc:
-            with ShieldCancellation():
-                with Trace("response_closed", logger, request) as trace:
-                    self._response_closed()
+            with Trace("response_closed", logger, request) as trace:
+                sync_cancel_shield(self._response_closed)
             raise exc
 
     # Sending the request...
@@ -344,8 +343,7 @@ def __iter__(self) -> Iterator[bytes]:
             # If we get an exception while streaming the response,
             # we want to close the response (and possibly the connection)
             # before raising that exception.
-            with ShieldCancellation():
-                self.close()
+            sync_cancel_shield(self.close)
             raise exc
 
     def close(self) -> None:
diff --git a/httpcore/_sync/http2.py b/httpcore/_sync/http2.py
index 1ee4bbb34..ea0b02b7d 100644
--- a/httpcore/_sync/http2.py
+++ b/httpcore/_sync/http2.py
@@ -17,7 +17,7 @@
     RemoteProtocolError,
 )
 from .._models import Origin, Request, Response
-from .._synchronization import Lock, Semaphore, ShieldCancellation
+from .._synchronization import Lock, Semaphore, sync_cancel_shield
 from .._trace import Trace
 from .interfaces import ConnectionInterface
 
@@ -108,8 +108,7 @@ def handle_request(self, request: Request) -> Response:
                     with Trace("send_connection_init", logger, request, kwargs):
                         self._send_connection_init(**kwargs)
                 except BaseException as exc:
-                    with ShieldCancellation():
-                        self.close()
+                    sync_cancel_shield(self.close)
                     raise exc
 
                 self._sent_connection_init = True
@@ -160,11 +159,12 @@ def handle_request(self, request: Request) -> Response:
                     "stream_id": stream_id,
                 },
             )
-        except BaseException as exc:  # noqa: PIE786
-            with ShieldCancellation():
-                kwargs = {"stream_id": stream_id}
-                with Trace("response_closed", logger, request, kwargs):
-                    self._response_closed(stream_id=stream_id)
+        except BaseException as exc:
+            kwargs = {"stream_id": stream_id}
+            with Trace("response_closed", logger, request, kwargs):
+                sync_cancel_shield(
+                    lambda: self._response_closed(stream_id=stream_id)
+                )
 
             if isinstance(exc, h2.exceptions.ProtocolError):
                 # One case where h2 can raise a protocol error is when a
@@ -577,8 +577,7 @@ def __iter__(self) -> typing.Iterator[bytes]:
             # If we get an exception while streaming the response,
             # we want to close the response (and possibly the connection)
             # before raising that exception.
-            with ShieldCancellation():
-                self.close()
+            sync_cancel_shield(self.close)
             raise exc
 
     def close(self) -> None:
diff --git a/httpcore/_synchronization.py b/httpcore/_synchronization.py
index 9619a3983..ccf22fafa 100644
--- a/httpcore/_synchronization.py
+++ b/httpcore/_synchronization.py
@@ -1,10 +1,12 @@
+import asyncio
+import sys
 import threading
 from types import TracebackType
-from typing import Optional, Type
+from typing import Any, Callable, Coroutine, Literal, Optional, Protocol, Type
 
 from ._exceptions import ExceptionMapping, PoolTimeout, map_exceptions
 
-# Our async synchronization primatives use either 'anyio' or 'trio' depending
+# Our async synchronization primitives use either 'asyncio' or 'trio' depending
 # on if they're running under asyncio or trio.
 
 try:
@@ -18,30 +20,49 @@
     anyio = None  # type: ignore
 
 
-def current_async_library() -> str:
+if sys.version_info >= (3, 11):  # pragma: nocover
+    import asyncio as asyncio_timeout
+else:  # pragma: nocover
+    import async_timeout as asyncio_timeout
+
+
+AsyncBackend = Literal["asyncio", "trio"]
+
+
+def current_async_backend() -> AsyncBackend:
     # Determine if we're running under trio or asyncio.
     # See https://sniffio.readthedocs.io/en/latest/
     try:
         import sniffio
     except ImportError:  # pragma: nocover
-        environment = "asyncio"
+        backend: AsyncBackend = "asyncio"
     else:
-        environment = sniffio.current_async_library()
+        backend = sniffio.current_async_library()  # type: ignore[assignment]
 
-    if environment not in ("asyncio", "trio"):  # pragma: nocover
-        raise RuntimeError("Running under an unsupported async environment.")
+    if backend not in ("asyncio", "trio"):  # pragma: nocover
+        raise RuntimeError("Running under an unsupported async backend.")
 
-    if environment == "asyncio" and anyio is None:  # pragma: nocover
+    if backend == "asyncio" and anyio is None:  # pragma: nocover
         raise RuntimeError(
             "Running with asyncio requires installation of 'httpcore[asyncio]'."
         )
 
-    if environment == "trio" and trio is None:  # pragma: nocover
+    if backend == "trio" and trio is None:  # pragma: nocover
         raise RuntimeError(
             "Running with trio requires installation of 'httpcore[trio]'."
         )
 
-    return environment
+    return backend
+
+
+class _LockProto(Protocol):
+    async def acquire(self) -> Any: ...
+    def release(self) -> None: ...
+
+
+class _EventProto(Protocol):
+    def set(self) -> None: ...
+    async def wait(self) -> Any: ...
 
 
 class AsyncLock:
@@ -53,28 +74,25 @@ class AsyncLock:
     """
 
     def __init__(self) -> None:
-        self._backend = ""
+        self._lock: Optional[_LockProto] = None
 
     def setup(self) -> None:
         """
         Detect if we're running under 'asyncio' or 'trio' and create
         a lock with the correct implementation.
         """
-        self._backend = current_async_library()
-        if self._backend == "trio":
-            self._trio_lock = trio.Lock()
-        elif self._backend == "asyncio":
-            self._anyio_lock = anyio.Lock()
+        if current_async_backend() == "trio":
+            self._lock = trio.Lock()
+        else:
+            # Note: asyncio.Lock has better performance characteristics than anyio.Lock
+            # https://github.com/encode/httpx/issues/3215
+            self._lock = asyncio.Lock()
 
     async def __aenter__(self) -> "AsyncLock":
-        if not self._backend:
+        if self._lock is None:
             self.setup()
-
-        if self._backend == "trio":
-            await self._trio_lock.acquire()
-        elif self._backend == "asyncio":
-            await self._anyio_lock.acquire()
-
+        lock: _LockProto = self._lock  # type: ignore[assignment]
+        await lock.acquire()
         return self
 
     async def __aexit__(
@@ -83,10 +101,8 @@ async def __aexit__(
         exc_value: Optional[BaseException] = None,
         traceback: Optional[TracebackType] = None,
     ) -> None:
-        if self._backend == "trio":
-            self._trio_lock.release()
-        elif self._backend == "asyncio":
-            self._anyio_lock.release()
+        lock: _LockProto = self._lock  # type: ignore[assignment]
+        lock.release()
 
 
 class AsyncThreadLock:
@@ -112,117 +128,95 @@ def __exit__(
 class AsyncEvent:
     def __init__(self) -> None:
         self._backend = ""
+        self._event: Optional[_EventProto] = None
 
     def setup(self) -> None:
         """
         Detect if we're running under 'asyncio' or 'trio' and create
         a lock with the correct implementation.
         """
-        self._backend = current_async_library()
+        self._backend = current_async_backend()
         if self._backend == "trio":
-            self._trio_event = trio.Event()
-        elif self._backend == "asyncio":
-            self._anyio_event = anyio.Event()
+            self._event = trio.Event()
+        else:
+            # Note: asyncio.Event has better performance characteristics than anyio.Event
+            self._event = asyncio.Event()
 
     def set(self) -> None:
-        if not self._backend:
+        if self._event is None:
             self.setup()
-
-        if self._backend == "trio":
-            self._trio_event.set()
-        elif self._backend == "asyncio":
-            self._anyio_event.set()
+        event: _EventProto = self._event  # type: ignore[assignment]
+        event.set()
 
     async def wait(self, timeout: Optional[float] = None) -> None:
-        if not self._backend:
+        if self._event is None:
             self.setup()
+        event: _EventProto = self._event  # type: ignore[assignment]
 
         if self._backend == "trio":
             trio_exc_map: ExceptionMapping = {trio.TooSlowError: PoolTimeout}
             timeout_or_inf = float("inf") if timeout is None else timeout
             with map_exceptions(trio_exc_map):
                 with trio.fail_after(timeout_or_inf):
-                    await self._trio_event.wait()
-        elif self._backend == "asyncio":
-            anyio_exc_map: ExceptionMapping = {TimeoutError: PoolTimeout}
-            with map_exceptions(anyio_exc_map):
-                with anyio.fail_after(timeout):
-                    await self._anyio_event.wait()
+                    await event.wait()
+        else:
+            asyncio_exc_map: ExceptionMapping = {
+                asyncio.exceptions.TimeoutError: PoolTimeout
+            }
+            with map_exceptions(asyncio_exc_map):
+                async with asyncio_timeout.timeout(timeout):
+                    await event.wait()
 
 
 class AsyncSemaphore:
     def __init__(self, bound: int) -> None:
         self._bound = bound
-        self._backend = ""
+        self._semaphore: Optional[_LockProto] = None
 
     def setup(self) -> None:
         """
         Detect if we're running under 'asyncio' or 'trio' and create
         a semaphore with the correct implementation.
         """
-        self._backend = current_async_library()
-        if self._backend == "trio":
-            self._trio_semaphore = trio.Semaphore(
-                initial_value=self._bound, max_value=self._bound
-            )
-        elif self._backend == "asyncio":
-            self._anyio_semaphore = anyio.Semaphore(
+        if current_async_backend() == "trio":
+            self._semaphore = trio.Semaphore(
                 initial_value=self._bound, max_value=self._bound
             )
+        else:
+            # Note: asyncio.BoundedSemaphore has better performance characteristics than anyio.Semaphore
+            self._semaphore = asyncio.BoundedSemaphore(self._bound)
 
     async def acquire(self) -> None:
-        if not self._backend:
+        if self._semaphore is None:
             self.setup()
-
-        if self._backend == "trio":
-            await self._trio_semaphore.acquire()
-        elif self._backend == "asyncio":
-            await self._anyio_semaphore.acquire()
+        semaphore: _LockProto = self._semaphore  # type: ignore[assignment]
+        await semaphore.acquire()
 
     async def release(self) -> None:
-        if self._backend == "trio":
-            self._trio_semaphore.release()
-        elif self._backend == "asyncio":
-            self._anyio_semaphore.release()
+        semaphore: _LockProto = self._semaphore  # type: ignore[assignment]
+        semaphore.release()
 
 
-class AsyncShieldCancellation:
-    # For certain portions of our codebase where we're dealing with
-    # closing connections during exception handling we want to shield
-    # the operation from being cancelled.
-    #
-    # with AsyncShieldCancellation():
-    #     ... # clean-up operations, shielded from cancellation.
-
-    def __init__(self) -> None:
-        """
-        Detect if we're running under 'asyncio' or 'trio' and create
-        a shielded scope with the correct implementation.
-        """
-        self._backend = current_async_library()
-
-        if self._backend == "trio":
-            self._trio_shield = trio.CancelScope(shield=True)
-        elif self._backend == "asyncio":
-            self._anyio_shield = anyio.CancelScope(shield=True)
-
-    def __enter__(self) -> "AsyncShieldCancellation":
-        if self._backend == "trio":
-            self._trio_shield.__enter__()
-        elif self._backend == "asyncio":
-            self._anyio_shield.__enter__()
-        return self
-
-    def __exit__(
-        self,
-        exc_type: Optional[Type[BaseException]] = None,
-        exc_value: Optional[BaseException] = None,
-        traceback: Optional[TracebackType] = None,
-    ) -> None:
-        if self._backend == "trio":
-            self._trio_shield.__exit__(exc_type, exc_value, traceback)
-        elif self._backend == "asyncio":
-            self._anyio_shield.__exit__(exc_type, exc_value, traceback)
+async def async_cancel_shield(
+    shielded: Callable[[], Coroutine[Any, Any, None]],
+) -> None:
+    if current_async_backend() == "trio":
+        with trio.CancelScope(shield=True):
+            await shielded()
+    else:
+        inner_task = asyncio.create_task(shielded())
+        retry = False
+        while True:
+            try:
+                await asyncio.shield(inner_task)
+                break
+            except asyncio.CancelledError:
+                if inner_task.done() or retry:
+                    break
+                # We may get multiple cancellations.
+                # Retry once to get inner_task finished here by best effort.
+                retry = True
+                continue
 
 
 # Our thread-based synchronization primitives...
@@ -301,17 +295,8 @@ def release(self) -> None:
         self._semaphore.release()
 
 
-class ShieldCancellation:
-    # Thread-synchronous codebases don't support cancellation semantics.
-    # We have this class because we need to mirror the async and sync
-    # cases within our package, but it's just a no-op.
-    def __enter__(self) -> "ShieldCancellation":
-        return self
-
-    def __exit__(
-        self,
-        exc_type: Optional[Type[BaseException]] = None,
-        exc_value: Optional[BaseException] = None,
-        traceback: Optional[TracebackType] = None,
-    ) -> None:
-        pass
+# Thread-synchronous codebases don't support cancellation semantics.
+# We have this class because we need to mirror the async and sync
+# cases within our package, but it's just a no-op.
+def sync_cancel_shield(fn: Callable[[], None]) -> None:
+    fn()
diff --git a/pyproject.toml b/pyproject.toml
index 85c787402..7a9fea162 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -31,6 +31,7 @@ classifiers = [
 dependencies = [
     "certifi",
     "h11>=0.13,<0.15",
+    "async-timeout==4.*; python_version < '3.11'",
 ]
 
 [project.optional-dependencies]
diff --git a/requirements.txt b/requirements.txt
index 0f395f96d..6b923a906 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -3,21 +3,21 @@
 # Docs
 mkdocs==1.6.0
 mkdocs-autorefs==1.0.1
-mkdocs-material==9.5.20
+mkdocs-material==9.5.25
 mkdocs-material-extensions==1.3.1
-mkdocstrings[python-legacy]==0.25.0
+mkdocstrings[python-legacy]==0.25.1
 jinja2==3.1.4
 
 # Packaging
 build==1.2.1
-twine==5.0.0
+twine==5.1.0
 
 # Tests & Linting
-coverage[toml]==7.5.0
-ruff==0.4.2
+coverage[toml]==7.5.3
+ruff==0.4.7
 mypy==1.10.0
 trio-typing==0.10.0
-pytest==8.2.0
+pytest==8.2.1
 pytest-httpbin==2.0.0
 pytest-trio==0.8.0
 werkzeug<2.1  # See: https://github.com/psf/httpbin/issues/35
diff --git a/scripts/unasync.py b/scripts/unasync.py
index 5a5627d71..b81bc638e 100644
--- a/scripts/unasync.py
+++ b/scripts/unasync.py
@@ -24,6 +24,7 @@
     ('@pytest.mark.anyio', ''),
     ('@pytest.mark.trio', ''),
     ('AutoBackend', 'SyncBackend'),
+    ('async_cancel_shield', 'sync_cancel_shield'),
 ]
 COMPILED_SUBS = [
     (re.compile(r'(^|\b)' + regex + r'($|\b)'), repl)