Added Performance CHANGELOG.md section

Author: jonmmease

Diff for: CHANGELOG.md

@@ -10,6 +10,13 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   makes it possible to skip hover information for some arguments or to change  
   the formatting of hover informatiom [#2377](https://github.com/plotly/plotly.py/pull/2377).
 
+### Performance
+This version includes several performance improvements ([#2368](https://github.com/plotly/plotly.py/pull/2368), [#2403](https://github.com/plotly/plotly.py/pull/2403)).
+ 
+ - Child graph objects (e.g. `figure.layout.xaxis`) are no longer created eagerly during graph object construction. Instead, they are created lazily the first time the property is accessed.
+ - Property validation is now disabled for select internal operations.
+ - When used with Python 3.7 and above, ploty.py now takes advantage of [PEP-562](https://www.python.org/dev/peps/pep-0562/) to perform submodule imports lazily.  This dramatically improves import times.
+
 ## [4.6] - 2020-03-31
 
 ### Updated