Document other performance implication for __slots__ (GH-11974) (GH-11975)

(cherry picked from commit 7463884f6993edcd92bec56213ee1959034fd31c)

Co-authored-by: Raymond Hettinger <rhettinger@users.noreply.github.com>
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
index f31ddb1..22c9b41 100644
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -1725,6 +1725,7 @@
 (unless explicitly declared in *__slots__* or available in a parent.)
 
 The space saved over using *__dict__* can be significant.
+Attribute lookup speed can be significantly improved as well.
 
 .. data:: object.__slots__