WordPress heartbeat is out of control

I’ve just migrated our WordPress hosting to a newer dedicated server, configuration is the same as the old server, but the heartbeat is way out of control. I see at least 2-4 POST requests a second for admin-ajax.php. The backend is loading insanely slow on some pages, while loading perfectly fine on others. The slow loading pages include the general settings, the dashboard, W3 Total Cache dashboard/settings, and tools.

Here is a screenshot of how many requests happened in a single minute:
https://prntscr.com/ki7tls

And every single requests is exactly the same, sending this:
https://prntscr.com/ki7u0i

I have tried disabling all plugins, switching to the default theme, and clearing all caches (mod_pagespeed & W3TC), but that didn’t fix anything.

The page I need help with: [log in to see the link]