Update CHANGELOG.md

Author: oleibman

CHANGELOG.md

@@ -29,7 +29,9 @@ and this project adheres to [Semantic Versioning](https://semver.org). Thia is a
 
 ### Fixed
 
-- Nothing yet.
+- Performance improvement when working with large amounts of cells. [Issue #4607](https://github.com/PHPOffice/PhpSpreadsheet/issues/4607) [PR #4609](https://github.com/PHPOffice/PhpSpreadsheet/pull/4609)
+- Minor improvements to Calculation coverage. [PR #4624](https://github.com/PHPOffice/PhpSpreadsheet/pull/4624)
+- Conditional formatting in extLst. [Issue #4629](https://github.com/PHPOffice/PhpSpreadsheet/issues/4629) [PR #4633](https://github.com/PHPOffice/PhpSpreadsheet/pull/4633)
 
 ## 2025-09-03 - 5.1.0