docs: add callback registration optimization to v1.81.9 release notes (#20681)

* docs: add callback registration optimization to v1.81.9 release notes

* Update v1.81.9.md

---------

Co-authored-by: Alexsander Hamir <alexsanderhamirgomesbaptista@gmail.com>

docs/my-website/release_notes/v1.81.9.md

@@ -48,6 +48,13 @@ pip install litellm==1.81.9
 - **UI Team Soft Budget Alerts** - [Set soft budgets on teams and receive email alerts when spending crosses the threshold — without blocking requests](../../docs/proxy/ui_team_soft_budget_alerts)
 - **Performance Optimizations** - Multiple performance improvements including ~40% Prometheus CPU reduction, LRU caching, and optimized logging paths
 - **LiteLLM Observatory** - [Automated 24-hour load tests](../../blog/litellm-observatory)
+- **30% Faster Request Processing for Callback-Heavy Deployments** - [Performance improvement for callback heavy deployments][PR #20354](https://github.com/BerriAI/litellm/pull/20354)
+
+---
+
+## 30% Faster Request Processing for Callback-Heavy Deployments
+
+ If you use logging callbacks like Langfuse, Datadog, or Prometheus, every request was paying an unnecessary cost: three loops that re-sorted your callbacks on every single request, even though the callback list hadn't changed. The more callbacks you had configured, the more time was wasted. We moved this work to happen once at startup instead of on every request. For deployments with the default callback set, this is a ~30% speedup in request setup. For deployments with many callbacks configured, the improvement is even larger.
 
 ---