Fixed #29994 -- Added warning about performance of FileBasedCache with a large number of files.

Co-authored-by: sheenarbw <699166+sheenarbw@users.noreply.github.com>
Co-authored-by: Mariusz Felisiak <felisiak.mariusz@gmail.com>

docs/topics/cache.txt

@@ -375,6 +375,14 @@ the user ``apache``.
     content, which your site will trust, but also remotely execute arbitrary
     code, as the data is serialized using :mod:`pickle`.
 
+.. warning::
+
+    Filesystem caching may become slow when storing a large number of files. If
+    you run into this problem, consider using a different caching mechanism.
+    You can also subclass
+    :source:`FileBasedCache <django/core/cache/backends/filebased.py>` and
+    improve the culling strategy.
+
 .. _local-memory-caching:
 
 Local-memory caching