I have recently (in the last few weeks) found that calculating slot times and availabilities takes a long time on Idlewild.
This concerns changing times, offsets, speed override, etc. while scheduling in the Flight Plan section. Every time I make a change it takes several seconds to take effect. That might sound trivial, but if you want to schedule a bunch of planes it does get annoying - especially when scheduling in airports where slots are scarce.
I have tried this when hooked up to a university backbone, so I doubt that it's my connection. Moreover, other parts of the site seem to work just fine.