diff --git a/developer-workflow/development-cycle.rst b/developer-workflow/development-cycle.rst
index af198a400..88ffb36d6 100644
--- a/developer-workflow/development-cycle.rst
+++ b/developer-workflow/development-cycle.rst
@@ -75,7 +75,9 @@ releases; the terms are used interchangeably. These releases have a
 **micro version** number greater than zero.
 
 The only changes allowed to occur in a maintenance branch without debate are
-bug fixes, test improvements, and edits to the documentation.
+bug fixes, test improvements, and edits to the documentation. Performance
+improvements are not among them: an optimization can introduce a regression,
+and a regression in a maintenance release is painful to track down and undo.
 Also, a general rule for maintenance branches is that compatibility
 must not be broken at any point between sibling micro releases (3.12.1, 3.12.2,
 etc.).  For both rules, only rare exceptions are accepted and **must** be