Calendar.app extremely slow in El Capitan. Rebuild cache? Other solution? After updating to El Capitan, my friend found her Calendar.app runs extremely slowly. Clicking the button to move to the next week's display results in spinning beach ball (the new one). Beach ball spins for something like half a minute. Makes the app practically unusable for someone booking appointments while the customer waits.

The same calendars shared over iCloud run nearly instantaneously on an iPad Calendar app (iOS 9.0.2). So the problem seems to be Mac-only. 

All other apps run well. Only Calendar is beach-ball-spinning slow.

Calendar.app was already annoyingly slow in Mountain Lion &amp; Yosemite, but is much worse in El Capitan. This computer has been upgraded through the years across these OSes.

Perhaps crud has accumulated in the Mac's local files for these calendars. Is there some way to rebuild or replace the Mac's cache of these calendars and get fresh copies from iCloud?

Any other workarounds or solutions?
I guess I had the same problem and just found a solution - at least in my case it helped a lot. Not as fast as it was in the past but much faster than it was after updating to El Capitan. And no more spinning balls ;)

Close Calendar.app, go to ~/Library/Preferences/ in your home folder and delete com.apple.iCal.plist.

Good luck!