Hum, you can try logging on one of these computers and as soon as you logged on, press Ctrl+Alt+Del, then Esc, without needing to access the task manager.
It's possible that when you escape you will arrive directly to the desktop, much faster than usually. If it is the case, try the regedit fix in the link I posted.
In my case, I wasn't using the solid-color background anymore, but was still suffering the 30 seconds wait. As soon as I applied the fix it was gone.
Hope this helps, and good luck with your problem!
