In WordPress 3.6 the WordPress Heartbeat API was introduced, which allows WordPress to communicate between the web-browser and the server. This improved upon session management, revision tracking, and auto-saving.
The WordPress Heartbeat API uses
admin-ajax.php to run AJAX calls from the web-browser. You have probably seen this when running speed tests. This can cause high CPU usage and a lot of PHP calls. For example, if you leave your dashboard open it will keep sending POST requests to this file on a regular interval.
There is a free WordPress plugin called heartbeat control which allows you to easily manage the frequency of the WordPress heartbeat API.