If you notice that you are having an excessive amount of admin-ajax.php requests, the WordPress Heartbeat API can be disabled to prevent this type of activity from happening automatically.
By default WordPress uses the Heartbeat API to manage things such as post locking so only one admin can edit a post at once, it's also used for auto saving. Going forward the API could be used more and more by WordPress developers to handle certain tasks, so keep this in mind if you choose to disable it.