I took all of my appointments from my main calendar previous to 2002 and put them in a separate file. I don't use that file (for now). My main calendar file has shrunk from 568K to 264K to 20K. iCal is much faster now.
However, iCal did not like that I hand-modified the files. The indexes (~/Library/Caches/) were busted. I couldn't figure out how to recreate them, and iCal seemed uninterested in recreating them automatically. I exported the data, deleted the calendars, created new calendars, and imported the data again. Harumph.
I keep finding a lot more little problems (sometimes I can't Get Info for a subscribed calendar; sometimes I lose metadata [when to refresh, even that it is a subscribed calendar at all]), but it is to the point where iCal is quite usable.
And someone's put up NHL calendars, filling a horrific void left by Apple (apparently someone thinks there are more French soccer fans than NHL fans). Harumph!