This is something I have noticed for quite a while. That is after an update the time to login on the first character is normally fairly quick, but the time used to logout and login to a second and subsequent characters is really significant 5 to 8 minutes (or more).
I believe the client software is updating itself and for some reason a logout and login results in something not very efficient going on, maybe it gets stuck and there are timeout waits, conjecture is all I have, ...
My strategy post an update is to login to the first character then when I am done and want to log into another I Quit the game and restart from the Launcher although I do normally wait until the second character does not login for a few mins and then kill the process and restart from the launcher. I am always hoping for it to be OK
With this Quit/Restart method I get a good response for character login.
Maybe ZOS could implement something in the code so that whatever is needed to happen(update/refresh) at the client end after an update, a change of character does not cause significant load screen time penalties.