diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index a7a7c85..796ac48 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -15,6 +15,10 @@ Release Notes
   using input catalogs that failed to align in the expanded reference
   catalog. [#195]
 
+- Reduce memory & compute needed by _xy_2dhist by pruning distant
+  pairs with a kdtree.  This is a purely internal change that does not
+  affect the results of the algorithm.  [#196]
+
 
 0.8.5 (30-November-2023)
 ========================
diff --git a/tweakwcs/matchutils.py b/tweakwcs/matchutils.py
index ad4aa04..d846494 100644
--- a/tweakwcs/matchutils.py
+++ b/tweakwcs/matchutils.py
@@ -278,9 +278,6 @@ def __call__(self, refcat, imcat, tp_pscale=1.0, tp_units=None, **kwargs):
 
 
 def _xy_2dhist(imgxy, refxy, r):
-    # This code replaces the C version (arrxyzero) from carrutils.c
-    # It is about 5-8 times slower than the C version.
-
     # trim to only pairs within (r+0.5) * np.sqrt(2) using a kdtree
     # to avoid computing differences for many widely separated pairs.
     kdtree = spatial.KDTree(refxy)
@@ -290,7 +287,7 @@ def _xy_2dhist(imgxy, refxy, r):
     if len(mi) > 0:
         mr = np.concatenate([n for n in neighbors if len(n) > 0])
     else:
-        mr = mi.copy()
+        mr = mi
 
     dx = imgxy[mi, 0] - refxy[mr, 0]
     dy = imgxy[mi, 1] - refxy[mr, 1]