A while back wordpress changed their system to call admin-ajax.php regularly, a little too regularly. This is known as the wordpress heartbeat. The process can slow your website response time down considerably for both visitors and admin (especially if you have more than one person logged on at a time).
The easiest way to fix this is with the heartbeat plugin, available from the repository. There are other ways to fix, including an addition to your themes function file, but using the plugin is the quickest way.
If you’re seeing bottlenecks when using a speed test, check for plugins that are calling admin-ajax.php in your speed test or waterfall report.