iCal Slow after El Capitan 10.11 Upgrade
After upgrading to El Capitan, iCal was extremely slow, with lots of spinning beach balls and long wait times each time I was trying to edit a calendar entry. Checking in the Apple forums revealed that others were having similar issues, along with the occasional iCal crash.
The solution that worked here:
1. Close Calendar.
2. Finder > Go (while holding down option key) > Library > Preferences
3. Find com.apple.iCal.plist and drag it to the desktop.
4. Reopen Calendar.
No more beach balls!