Disable the Heartbeat API Except in the Post Editor
Keeps the Heartbeat API (autosave and post locking) in the editor but turns it off on every other admin and front-end page to save server requests.
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Find What Is Calling admin-ajax.php
Counts admin-ajax.php requests per action (and whether they come from the front end or the dashboard) for the current day and the previous six, and shows the busiest ones under Tools > AJAX Usage.