I know this is an old thread, but I found when looking for solutions to my problem, and it might help someone else.
I had problems since the install of SSMS v18.10 / 15.0.18390.0. The splash screen would come up after 2 or 3 seconds, but the login prompt would take 20-30 seconds to open.
I tried (unsuccessfully) the "publisher certificate revocation" solution, the "encrypt connection / trust server certificate" solution, the "empty environment" solution; all to no avail. I reverted all of those changes before the step that worked.
I edited the properties of the shortcut to SSMS and checked "Run as Administrator" (in the shortcut). It causes the elevation prompt to be shown, but the splash comes up quickly with the login prompt. Time: 1.5 seconds.
On a local network, the table refresh and job step editing went from 10 seconds to less than 1 second.
I have no idea why this would be related to elevated permissions, but apparently it is, at least in my case.
I hope this helps someone else.