Smush Pro, Multisite and admin-ajax.php

We've noticed a large delay/lag on some sites within our multisite when uploading images, or viewing the media library. The site is incredibly slow - and using Google Chrome's developer tools - we're seeing a wait on admin-ajax.php of up to 2 minutes.

We've done a bunch of trial and error (deactivating plugins and themes) - and deactivating Smush Pro immediately fixed the problem. Problem returned when we reactivated the plugin.

Have you had any