⚡ Quick Fix (TL;DR)

The Culprit: A REST API timeout got initially treated as a general "site is slow" problem, prompting checks across Action Scheduler queues, cron events, file permissions, and caching settings — most of which came back clean and cost real time to rule out.

The Fix: The single most useful step was deactivating plugins one at a time and re-checking Site Health after each, which isolated the failure to one specific plugin's one specific REST endpoint (real-queue/v1/jobs). That turned a vague complaint into a concrete, researchable error. When general slowness and a specific error coexist, isolate the specific error first — the general feeling of slowness is often just a symptom of that one narrow cause, not a separate problem to solve on its own.