In WordPress 3.6, an API was introduced which keeps track and manages sessions as well as posts and pages revisions. This function acts like a pulse or a ‘Heartbeat’ – It requires your server to work slightly more in doing so, making it possible to cause performance issues.
Every pulse is around 95/100 Bytes in size, not much, but, the pulse runs every 15 seconds, so if you have a complex site, that is already using extensively the server the Heartbeat Pulse can add stress to your server.
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