Add changelog entry

Signed-off-by: Finn Carroll <[email protected]>

Author: finnegancarroll

CHANGELOG.md

@@ -18,6 +18,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Removed
 
 ### Fixed
+- Fix slow performance of FeatureFlag checks ([#17611](https://github.com/opensearch-project/OpenSearch/pull/17611))
 
 ### Security