Apple - Calendar.app extremely slow in El Capitan. Rebuild cache? Other solution?

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!

I solved my slow calendar by turning off Birthday calendars, Holiday Calendar, and Siri Found in Apps Calendar in the Calendar preferences.

I noticed an immediate improvement in speed. I restarted Calendar and the speed improved even more.

I suspect the slowness came from the birthday calendar feature. I have 5,787 contacts and that causes slowness elsewhere. I noticed Calendar app RAM usage went from about 300MB to about 100MB after turning off the special calendars.

Stefan's answer to Close Calendar.app, go to ~/Library/Preferences/ in your home folder and delete com.apple.iCal.plist helped initially, but things got slow again and another delete of the plist didn't help.