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..b3bb52c67e 100644
--- a/hypothesis-python/src/hypothesis/internal/reflection.py
+++ b/hypothesis-python/src/hypothesis/internal/reflection.py
@@ -27,8 +27,9 @@
 from random import _inst as global_random_instance
 from tokenize import COMMENT, detect_encoding, generate_tokens, untokenize
 from types import ModuleType
-from typing import TYPE_CHECKING, Any, Callable
+from typing import TYPE_CHECKING, Any, Callable, MutableMapping
 from unittest.mock import _patch as PatchType
+from weakref import WeakKeyDictionary
 
 from hypothesis.errors import HypothesisWarning
 from hypothesis.internal.compat import PYPY, is_typed_named_tuple
@@ -39,6 +40,7 @@
     from hypothesis.strategies._internal.strategies import T
 
 READTHEDOCS = os.environ.get("READTHEDOCS", None) == "True"
+LAMBDA_SOURCE_CACHE: MutableMapping[Callable, str] = WeakKeyDictionary()
 
 
 def is_mock(obj):
@@ -303,7 +305,7 @@ def visit_Lambda(self, node):
 SPACE_PRECEDES_CLOSE_BRACKET = re.compile(r" \)")
 
 
-def extract_lambda_source(f):
+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.
 
@@ -439,6 +441,17 @@ def extract_lambda_source(f):
     return source.strip()
 
 
+def extract_lambda_source(f):
+    try:
+        return LAMBDA_SOURCE_CACHE[f]
+    except KeyError:
+        pass
+
+    source = _extract_lambda_source(f)
+    LAMBDA_SOURCE_CACHE[f] = source
+    return source
+
+
 def get_pretty_function_description(f):
     if isinstance(f, partial):
         return pretty(f)
@@ -492,7 +505,7 @@ def repr_call(f, args, kwargs, *, reorder=True):
     if repr_len > 30000:
         warnings.warn(
             "Generating overly large repr. This is an expensive operation, and with "
-            f"a length of {repr_len//1000} kB is is unlikely to be useful. Use -Wignore "
+            f"a length of {repr_len//1000} kB is unlikely to be useful. Use -Wignore "
             "to ignore the warning, or -Werror to get a traceback.",
             HypothesisWarning,
             stacklevel=2,
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>"