OS X Calendar extremely slow
The first step is to make a new user account. 


Does calendar app itself contain the problem with no calendar data?


Then you can isolate things from the network. Pull any wired networks and power down WiFi and restart. Retest calendar in the test account (which presumably is always fast and responsive) and your account (which is slow now).

Once you've done that, you can enable networking, sign out of your account and add your calendar source data one by one (if you maintain several calendars in OS X) to the test account to tell if your data is slowing the app.

Assuming you have a speedy calendar in the test account with your real calendar data - you can conclude that your caches for the data was the cause of the slowness. When that happens, I usually move everything over to a new account since the corruption in one app is almost always shared in another.

To make sure the disk isn't causing further issues:


delete the calendar data from the test account
make a full backup (if you like) as well as a Time Machine backup
restart the Mac to recovery HD to repair the volume with Disk Utility (or do a safe boot which runs the same filesystem check)
use the test account to remove the old account - keeping the files on disk (don't erase and don't make an image of the home folder when asked)
make the normal user account and then use that to move back the data from the old home folder