iCal very slow on each edit

I'm having this problem with iCal 5.0.1 and Lion on two different iMacs: a 2010 i7 and a 2008 Core 2 Duo. Both are up to date with system updates, etc.

The problem is this: each and every time I edit iCal -- new event, change an old event, edit a current event, etc -- iCal takes 10 to 30 seconds to update. I get the spinning beachball each time. Sometimes the menu bar title says 'iCal ... updating', sometimes not. Then the very NEXT mouse click in iCal, anywhere, gets me another spinning beachball for another 10 to 30 seconds.

This does NOT happen with iCal 5.0.1 on my brand-new MacBook Air running Lion (came installed). Both the iMacs were updated to Lion from SL, etc.

I can sync the calendars without problems between the three computers and with my iPhone 4 and iPad 2, all via iCloud. An event entered on one device shows up properly on the other devices.

The slow-down problem was happening when I was using MobileMe to sync but was not as bad; it's much worse now that I'm syncing with iCloud. Likewise, it occurred when the iMacs were running SL but not as bad; now much worse with Lion. On each iMac, I changed the 'refresh calendars' in the Accounts tab in preferences from 'push' to 'every five minutes' but that hasn't helped. I am syncing three calendars ('home', 'work', 'consulting') via iCloud, and one calendar on the Mac.

Each calendar has a number of events. I edited all the repeating events so that they all end at the end of 2012, wondering if perhaps repeating to infinity was a problem, but that hasn't helped.

On both iMacs, I have trashed the iCal.plist in my library preferences and then logged-out/logged-in, but that also hasn't helped.

Any suggestions? iCal is pretty much unusable on the iMacs.

Thanks in advance,

Steve