diff --git a/hypothesis-python/RELEASE.rst b/hypothesis-python/RELEASE.rst
new file mode 100644
index 0000000000..34de7f7271
--- /dev/null
+++ b/hypothesis-python/RELEASE.rst
@@ -0,0 +1,3 @@
+RELEASE_TYPE: patch
+
+This patch fixes a significant slowdown when using the :func:`~hypothesis.stateful.precondition` decorator in some cases, due to expensive repr formatting internally (:issue:`3963`).
diff --git a/hypothesis-python/src/RELEASE.rst b/hypothesis-python/src/RELEASE.rst
deleted file mode 100644
index 373954606d..0000000000
--- a/hypothesis-python/src/RELEASE.rst
+++ /dev/null
@@ -1,3 +0,0 @@
-RELEASE_TYPE: patch
-
-This patch cleans up some internal code.
diff --git a/hypothesis-python/src/hypothesis/internal/reflection.py b/hypothesis-python/src/hypothesis/internal/reflection.py
index c9ffa6eb2d..7f1f96a3ab 100644
--- a/hypothesis-python/src/hypothesis/internal/reflection.py
+++ b/hypothesis-python/src/hypothesis/internal/reflection.py
@@ -29,6 +29,7 @@
 from types import ModuleType
 from typing import TYPE_CHECKING, Any, Callable
 from unittest.mock import _patch as PatchType
+from functools import lru_cache
 
 from hypothesis.errors import HypothesisWarning
 from hypothesis.internal.compat import PYPY, is_typed_named_tuple
@@ -303,6 +304,7 @@ def visit_Lambda(self, node):
 SPACE_PRECEDES_CLOSE_BRACKET = re.compile(r" \)")
 
 
+@lru_cache(maxsize=1024)
 def extract_lambda_source(f):
     """Extracts a single lambda expression from the string source. Returns a
     string indicating an unknown body if it gets confused in any way.
diff --git a/hypothesis-python/tests/cover/test_reflection.py b/hypothesis-python/tests/cover/test_reflection.py
index 18b1262eff..3f9565e7be 100644
--- a/hypothesis-python/tests/cover/test_reflection.py
+++ b/hypothesis-python/tests/cover/test_reflection.py
@@ -541,12 +541,10 @@ def test_required_args(target, args, kwargs, expected):
     assert required_args(target, args, kwargs) == expected
 
 
-# fmt: off
-pi = "π"; is_str_pi = lambda x: x == pi  # noqa: E702
-# fmt: on
-
-
 def test_can_handle_unicode_identifier_in_same_line_as_lambda_def():
+    # fmt: off
+    pi = "π"; is_str_pi = lambda x: x == pi  # noqa: E702
+    # fmt: on
     assert get_pretty_function_description(is_str_pi) == "lambda x: x == pi"
 
 
@@ -567,6 +565,9 @@ def test_does_not_crash_on_utf8_lambda_without_encoding(monkeypatch):
     # has to fall back to assuming it's ASCII.
 
     monkeypatch.setattr(reflection, "detect_encoding", None)
+    # fmt: off
+    pi = "π"; is_str_pi = lambda x: x == pi  # noqa: E702
+    # fmt: on
     assert get_pretty_function_description(is_str_pi) == "lambda x: <unknown>"