WordPress 3.6.0 introduced the Heartbeat API. It’s the backbone to some of the new features such as session management, better revision management, post locking, auto-saving and more. Unfortunately, it also introduced increased CPU usage when logged into the WordPress Dashboard.
The issue lies in the way the Heartbeat API works. When logged in, your browser sends a POST request to wp-admin/admin-ajax.php every 15 seconds when your tab is active and every 120 seconds when your tab is in the background. Since it’s sending a POST request, it’s spawning a new PHP process. This isn’t so much an issue if you have a single tab open but the problem lays for users who like to keep multiple tabs open at a time while logged in to the Dashboard. Take a look at the following POST request data: