Redis + Varnish + OpenSearch Tuning for Magento 2.4.x
Most Magento performance guides stop at FPM_CHILDREN and "enable Varnish." The real wins live in six config keys nobody talks about: Redis eviction policy, Varnish ESI hold-times, OpenSearch query clause limits, and a handful of header rules. Here is the diff per key, the ROI measured on a real 80k-SKU prod trace, and the safe-rollback notes.