I've been frustratingly living with iCal unresponsiveness for what seems like years. About two dozen times a day, some interaction with iCal (a click on any sort of control, element, or field) results in about a 20-second wait while iCal thinks about it, sometimes with a beach ball, sometimes without. If you Control-click the app in the dock while this is going on, you get the 'not responding' message. iCal's CPU usage sometimes correspondingly spikes in top. Sometimes iCal will even refuse to budge its way in front of any other windows.
Background: I use iCal all the time. I generate probably four to-dos a day and a couple new events a week (not counting the existing recurring dozen events per week). I probably edit to-dos and events at a rate of 20 times per day. I have four calendars at work (G5 intel Mac Pro 10.4), about 12 calendars at home (G4 Power Book 10.3), and about 13 calendars on my previous work computer (G4 Power Mac 10.4). All computers have the same problem, even though the only common calendar between them is the US Holidays calendar from Apple's website (and it's not set to auto-refresh). I don't subscribe to .Mac, nor have I ever. I run iSync at home, but not at work. I publish calendars from the home computer to a fourth computer with webdav configured. Each computer is different, but they all had the same problem with unresponsiveness. I had assumed corrupt calendar files -- until I started having the same problem on my new G5 with different calendars and no publishing or subscriptions (except US holidays).
I occasionally search for a solution to this problem, but have never found one using the search terms ical hang OR freeze OR "not responding", though I don't claim to be a power searcher. Today, I got frustrated enough to actually call Apple and resolve the issue once and for all and I think I may have found a solution.
Here's what I did:
Mac OS X Hints
http://hints.macworld.com/article.php?story=2007091913425840