Add speed 8 and 9 quality improvements to CHANGELOG

Bug: aomedia:401234702
Change-Id: Ia281df0fffc3237b6ed60f5f5cd62ece99054460

CHANGELOG

diff --git a/CHANGELOG b/CHANGELOG
index 34ea55f..fc7c6c8 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -7,6 +7,18 @@
   Therefore aom_roi_map_t is to be considered as a new struct added in
   this release and this change does not break ABI compatibility.
 
+  - Compression Efficiency Improvements
+    * Variance Boost is now enabled for tuning mode AOM_TUNE_IQ at
+      speeds 8 and 9 (2-5% SSIMULACRA 2 BD-Rate gains)
+    * Several quality/time tradeoff improvements and bug fixes for all
+      intra mode speeds 8 and 9.
+      * Up to 6.9% SSIMULACRA 2 BD-Rate gains for speed 8
+      * Up to 2.2% SSIMULACRA 2 BD-Rate gains for speed 9
+
+  - Bug Fixes
+    * b:421196988: all intra speed 8: overuse of palette mode
+      unnecessarily inflating file sizes
+
 2025-04-11 v3.12.1
   This release includes several bug fixes. This release is ABI
   compatible with the last release. See