on a virtual server of about 50 users, clicking on Edit Users and then trying to edit a specific user takes a few tries to actually load up in full, and until it does the page takes about 30 seconds to actually load, as if it's hitting a timeout. I'm attaching a screenshot of how it shows up initially, and how it is when loaded correctly on subsequent tries. It doesn't matter how many times I actually try to edit the user: even just trying again makes it load up correctly, as long as I give it enough time before trying again (usually 2-3 minutes). Once it's loaded correctly once, loading it again happens immediately for some time (haven't tested it, but could be hours).
It should be noted that the mailboxes are somewhat big; the machine is however high-end.