If you've been using a Mac for years, you probably have hundreds or thousands of old events stored in iCal. All of that legacy data can bog down iCal and make it slow. And if you're syncing your calendars with others using BusySync, you're probably slowing them down too.
You can significantly increase the performance of iCal by archiving these old events using iCal Archiver.
iCal Archiver is a free utility that copies old events to an archive calendar and deletes them from the source calendar. The archive calendar can then be exported for safe keeping, and deleted from iCal. Once the old events have been deleted, iCal's database should be much smaller and require less memory, resulting in faster performance.