Backend Slowness Possibly Related to Admin-Ajax.php and Jquery


We are experiencing a slow backend since updating to 4.7.2. At first it broke the site on so many levels so we quickly switched back to 4.6.3 - the last working version. Even though it solved some problems, like we have no problem on the front end and we can create new posts which we couldnt, after the update, we still experience slowness and timeouts on the backend.

When I tried to diagnose the problem, I notice it had something to do with the admin-ajax.php. When I checked on Google dev tools it appears that it has something to do with Jquery

Here is a screenshot:

I have tried the basics like changing to default theme, disabling all plugins to no avail. Is there anyone who can help me with this? I granted support permission. This admin-ajax thing is just my guess, it might be something else, but I'm positive it is somehow related to the ajax thing since most problems I've encountered were on pages where ajax feature was active. Heartbeat plugin doesn't help either. I also tried to load the Jquery from Google library which did not fix the problem either.

Please someone help me out on this and I'm willing to pay whatever it costs to the guy who can save us from this once and for all.

Thank you