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!