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.
Most Magento TTFB advice stops at "enable Varnish". After dragging one Magento 2.4.9 store from 1.8 s down to 180 ms, three fixes mattered: PHP-FPM pm.max_children sized to actual per-process memory, a Redis cache_id_prefix collision that two stores quietly shared, and Varnish ESI scoped to cart and wishlist blocks. Here is the real var/log/php-fpm.log, redis-cli MONITOR output, and varnishlog trace that pinned each one — with a table that separates what hits TTFB from what only hits LCP and INP.
