Syncing iCal really isn't enough to slow down a computer, even if there are a lot of changes.
Use spotlight (CMD+Spacebar) to search for "Activity Monitor" when the computer starts to slow down. Open that, sort by CPU usage, and find out what's actually slowing it down. If nothing is, then most likely you are out of RAM. You can see how much free RAM you have in Activity Monitor, so do that and if it's under 300 MB, reboot the computer or close some apps.