How big can IMAP accounts be and outlook still works OK?

I'm dealing with a friend's laptop that is really slow.  He has several imap accounts for his domains that he has hosted from gmail and set up in outlook 2010.  They seem to be huge.  The PSTs (for imap? Or is that for the local folders under that account?  there's 3 PSTs for different email accounts - 7GB, 2GB and 3GB and 3 more that are about 0.5GB.  For the last half hour, it's been syncing mail

the icon at the bottom right says '
progress 25%
Action loading Outlook Events
Item 785 out of 1143

(i'm thinking that's 785 of 1143 for 1 mail account and there's others it hasn't started yet, that it says it's 25% complete?).

What kinds of limits are there for email and imap?  I am only used to PSTs / POP mail.  You hear about hosted exchange coming with 25GB (max) mailboxes.  Exchange is kinda like IMAP, right?  What's the performance of a laptop w/ outlook and exchange mailbox at 15 - 20 - 25GB?  

Thanks!