upgrade notes: explain performance improvements 2.51 (#13287)

* upgrade notes: explain performance benefits

* add two prs

Author: valentijnscholten

docs/content/en/open_source/upgrading/2.51.md

@@ -5,6 +5,18 @@ weight: -20250902
 description: Helm chart changes and Postgres major version updates.
 ---
 
+## Performance improvements
+
+This release includes multiple improvements aimed at making DefectDojo faster, more scalable, and lighter on your database and workers.
+
+- Import and reimport are significantly more efficient: product grading is now orchestrated in batches using Celery chords, reducing the number of background tasks and database churn during large scans. This means faster imports and smoother post-processing on busy systems. See [PR 12914](https://github.com/DefectDojo/django-DefectDojo/pull/12914).
+- Query-count reductions and importer hot-path tuning: we trimmed unnecessary ORM calls and optimized how findings/endpoints are updated during (re)import. You should see noticeably quicker runs out of the box. See [PR 13182](https://github.com/DefectDojo/django-DefectDojo/pull/13182) and [PR 13152](https://github.com/DefectDojo/django-DefectDojo/pull/13152).
+- Smarter background task orchestration for product graing: less duplicate work and better scheduling during heavy operations, keeping the UI responsive while long jobs run. See [PR 12900](https://github.com/DefectDojo/django-DefectDojo/pull/12900).
+- Bulk tag addition for large batches: adds an internal method to add tags to many findings at once, performing tagging in batches (default 1,000) with only a few queries per batch. This replaces ~3 queries per finding with ~3 queries per batch, significantly reducing DB load during imports, reimports, and bulk edit. On a ~10k-findings sample, import time dropped from ~372s to ~190s. See [PR 13285](https://github.com/DefectDojo/django-DefectDojo/pull/13285).
+- Preparations for our switch to `django-pghistory` which provides more features and better performance compared to `django-auditlog`. See [PR 13169](https://github.com/DefectDojo/django-DefectDojo/pull/13169).
+
+No configuration changes are required—gains are automatic after upgrading.
+
 ## Helm Chart Changes
 
 This release introduces several important changes to the Helm chart configuration: