Since the last two updates we've had really poor performance on all of our WPLMS sites. Getting admin-ajax 504 errors in the backend VC post editors and anywhere on the front edit where ajax is being used (in quizzes and marking a page complete in units).
We are hosted with WPEngine so I would imagine they can handle a normal about of requests. We have also changed the heartbeat to 60.
Thanks,
Lawrence
Well 504 error is a gateway timeout error. Please check your max execution time and contact your server about this.