diff --git a/rinja_derive/src/config.rs b/rinja_derive/src/config.rs
index 91b62911..894de13c 100644
--- a/rinja_derive/src/config.rs
+++ b/rinja_derive/src/config.rs
@@ -6,15 +6,13 @@ use std::path::{Path, PathBuf};
 use std::sync::{Arc, OnceLock};
 use std::{env, fs};
 
-use once_map::sync::OnceMap;
 use parser::node::Whitespace;
 use parser::{ParseError, Parsed, Syntax, SyntaxBuilder};
 use proc_macro2::Span;
-use rustc_hash::FxBuildHasher;
 #[cfg(feature = "config")]
 use serde::Deserialize;
 
-use crate::{CRATE, CompileError, FileInfo};
+use crate::{CRATE, CompileError, FileInfo, OnceMap};
 
 #[derive(Debug)]
 pub(crate) struct Config {
@@ -77,23 +75,20 @@ impl Config {
         template_whitespace: Option<&str>,
         config_span: Option<Span>,
     ) -> Result<&'static Config, CompileError> {
-        static CACHE: ManuallyDrop<
-            OnceLock<OnceMap<OwnedConfigKey, &'static Config, FxBuildHasher>>,
-        > = ManuallyDrop::new(OnceLock::new());
-        CACHE.get_or_init(OnceMap::default).get_or_try_insert_ref(
+        static CACHE: ManuallyDrop<OnceLock<OnceMap<OwnedConfigKey, &'static Config>>> =
+            ManuallyDrop::new(OnceLock::new());
+        CACHE.get_or_init(OnceMap::default).get_or_try_insert(
             &ConfigKey {
                 source: source.into(),
                 config_path: config_path.map(Cow::Borrowed),
                 template_whitespace: template_whitespace.map(Cow::Borrowed),
             },
-            config_span,
-            ConfigKey::to_owned,
-            |config_span, key| {
-                let config = Config::new_uncached(*key, config_span)?;
+            |key| {
+                let config = Config::new_uncached(key.to_owned(), config_span)?;
                 let config = &*Box::leak(Box::new(config));
-                Ok((config, config))
+                Ok((config._key, config))
             },
-            |_, _, config| config,
+            |config| *config,
         )
     }
 }
@@ -234,7 +229,7 @@ impl Config {
 #[derive(Debug, Default)]
 pub(crate) struct SyntaxAndCache<'a> {
     syntax: Syntax<'a>,
-    cache: OnceMap<OwnedSyntaxAndCacheKey, Arc<Parsed>, FxBuildHasher>,
+    cache: OnceMap<OwnedSyntaxAndCacheKey, Arc<Parsed>>,
 }
 
 impl<'a> Deref for SyntaxAndCache<'a> {
@@ -281,28 +276,27 @@ impl<'a> SyntaxAndCache<'a> {
         source: Arc<str>,
         source_path: Option<Arc<Path>>,
     ) -> Result<Arc<Parsed>, ParseError> {
-        self.cache.get_or_try_insert_ref(
+        self.cache.get_or_try_insert(
             &SyntaxAndCacheKey {
                 source: Cow::Owned(source),
                 source_path: source_path.map(Cow::Owned),
             },
-            &self.syntax,
             |key| {
-                OwnedSyntaxAndCacheKey(SyntaxAndCacheKey {
+                let key = OwnedSyntaxAndCacheKey(SyntaxAndCacheKey {
                     source: Cow::Owned(Arc::clone(key.source.as_ref())),
                     source_path: key
                         .source_path
                         .as_deref()
                         .map(|v| Cow::Owned(Arc::clone(v))),
-                })
-            },
-            |syntax, key| {
-                let source = Arc::clone(key.source.as_ref());
-                let source_path = key.source_path.as_deref().map(Arc::clone);
-                let parsed = Arc::new(Parsed::new(source, source_path, syntax)?);
-                Ok((Arc::clone(&parsed), parsed))
+                });
+                let parsed = Parsed::new(
+                    Arc::clone(key.source.as_ref()),
+                    key.source_path.as_deref().map(Arc::clone),
+                    &self.syntax,
+                )?;
+                Ok((key, Arc::new(parsed)))
             },
-            |_, _, cached| Arc::clone(cached),
+            Arc::clone,
         )
     }
 }
diff --git a/rinja_derive/src/input.rs b/rinja_derive/src/input.rs
index dbf0e525..497f683f 100644
--- a/rinja_derive/src/input.rs
+++ b/rinja_derive/src/input.rs
@@ -6,7 +6,6 @@ use std::path::{Path, PathBuf};
 use std::sync::{Arc, OnceLock};
 
 use mime::Mime;
-use once_map::OnceMap;
 use parser::{Node, Parsed};
 use proc_macro2::Span;
 use rustc_hash::FxBuildHasher;
@@ -14,7 +13,7 @@ use syn::punctuated::Punctuated;
 use syn::spanned::Spanned;
 
 use crate::config::{Config, SyntaxAndCache};
-use crate::{CompileError, FileInfo, MsgValidEscapers};
+use crate::{CompileError, FileInfo, MsgValidEscapers, OnceMap};
 
 pub(crate) struct TemplateInput<'a> {
     pub(crate) ast: &'a syn::DeriveInput,
@@ -766,19 +765,16 @@ pub(crate) fn get_template_source(
     tpl_path: &Arc<Path>,
     import_from: Option<(&Arc<Path>, &str, &str)>,
 ) -> Result<Arc<str>, CompileError> {
-    static CACHE: OnceLock<OnceMap<Arc<Path>, Arc<str>, FxBuildHasher>> = OnceLock::new();
+    static CACHE: OnceLock<OnceMap<Arc<Path>, Arc<str>>> = OnceLock::new();
 
-    CACHE.get_or_init(OnceMap::default).get_or_try_insert_ref(
+    CACHE.get_or_init(OnceMap::default).get_or_try_insert(
         tpl_path,
-        (),
-        Arc::clone,
-        |(), tpl_path| match read_to_string(tpl_path) {
+        |tpl_path| match read_to_string(tpl_path) {
             Ok(mut source) => {
                 if source.ends_with('\n') {
                     let _ = source.pop();
                 }
-                let source = Arc::from(source);
-                Ok((Arc::clone(&source), source))
+                Ok((Arc::clone(tpl_path), Arc::from(source)))
             }
             Err(err) => Err(CompileError::new(
                 format_args!(
@@ -790,7 +786,7 @@ pub(crate) fn get_template_source(
                 }),
             )),
         },
-        |(), _, cached| Arc::clone(cached),
+        Arc::clone,
     )
 }
 
diff --git a/rinja_derive/src/lib.rs b/rinja_derive/src/lib.rs
index 9eb6d89f..187af0da 100644
--- a/rinja_derive/src/lib.rs
+++ b/rinja_derive/src/lib.rs
@@ -10,10 +10,12 @@ mod input;
 #[cfg(test)]
 mod tests;
 
-use std::borrow::Cow;
-use std::collections::HashMap;
+use std::borrow::{Borrow, Cow};
+use std::collections::hash_map::{Entry, HashMap};
 use std::fmt;
+use std::hash::{BuildHasher, Hash};
 use std::path::Path;
+use std::sync::Mutex;
 
 use config::{Config, read_config_file};
 use generator::{Generator, MapChain};
@@ -25,6 +27,7 @@ use proc_macro::TokenStream as TokenStream12;
 #[cfg(feature = "__standalone")]
 use proc_macro2::TokenStream as TokenStream12;
 use proc_macro2::{Span, TokenStream};
+use rustc_hash::FxBuildHasher;
 use syn::parse_quote_spanned;
 
 /// The `Template` derive macro and its `template()` attribute.
@@ -379,6 +382,42 @@ impl fmt::Display for MsgValidEscapers<'_> {
     }
 }
 
+#[derive(Debug)]
+struct OnceMap<K, V>([Mutex<HashMap<K, V, FxBuildHasher>>; 8]);
+
+impl<K, V> Default for OnceMap<K, V> {
+    fn default() -> Self {
+        Self(Default::default())
+    }
+}
+
+impl<K: Hash + Eq, V> OnceMap<K, V> {
+    // The API of this function was copied, and adapted from the `once_map` crate
+    // <https://crates.io/crates/once_map/0.4.18>.
+    fn get_or_try_insert<T, Q, E>(
+        &self,
+        key: &Q,
+        make_key_value: impl FnOnce(&Q) -> Result<(K, V), E>,
+        to_value: impl FnOnce(&V) -> T,
+    ) -> Result<T, E>
+    where
+        K: Borrow<Q>,
+        Q: Hash + Eq,
+    {
+        let shard_idx = (FxBuildHasher.hash_one(key) % self.0.len() as u64) as usize;
+        let mut shard = self.0[shard_idx].lock().unwrap();
+        Ok(to_value(if let Some(v) = shard.get(key) {
+            v
+        } else {
+            let (k, v) = make_key_value(key)?;
+            match shard.entry(k) {
+                Entry::Vacant(entry) => entry.insert(v),
+                Entry::Occupied(_) => unreachable!("key in map when it should not have been"),
+            }
+        }))
+    }
+}
+
 // This is used by the code generator to decide whether a named filter is part of
 // Rinja or should refer to a local `filters` module.
 const BUILT_IN_FILTERS: &[&str] = &[
diff --git a/rinja_derive_standalone/Cargo.toml b/rinja_derive_standalone/Cargo.toml
index f503b3d8..f0768a17 100644
--- a/rinja_derive_standalone/Cargo.toml
+++ b/rinja_derive_standalone/Cargo.toml
@@ -42,7 +42,6 @@ serde = { version = "1.0", optional = true, features = ["derive"] }
 memchr = "2"
 mime = "0.3"
 mime_guess = "2"
-once_map = { version = "0.4.18", default-features = false, features = ["std"] }
 proc-macro2 = "1"
 quote = "1"
 rustc-hash = "2.0.0"