docs: update release notes

Author: ellemouton

docs/release-notes/release-notes-0.20.0.md

@@ -127,8 +127,16 @@ reader of a payment request.
   to improve readability and maintainability of the code.
 
 ## Breaking Changes
+
 ## Performance Improvements
 
+* Let the [channel graph cache be populated 
+  asynchronously](https://github.com/lightningnetwork/lnd/pull/10065) on 
+  startup. While the cache is being populated, the graph is still available for 
+  queries, but all read queries will be served from the database until the cache 
+  is fully populated. This new behaviour can be opted out of via the new 
+  `--db.sync-graph-cache-load` option.
+
 ## Deprecations
 
 ### ⚠️ **Warning:** The following RPCs will be removed in release version **0.21**: