Hey there,
I myself and many of my guildies all got massive Frame drops when the pre-patch came out. I was stuck at 9 fps no matter what I did. I tried everything I could find to solve the issue. Turns out it was a very easy fix for me and many of my guildies. Try these steps out.
1. Completely exit the game.
2. In the Battle.net Desktop Application, click Options and select Show in Explorer (Windows) or Reveal in Finder (Mac).
3. Rename the Cache, Interface, and WTF folders to CacheOld, InterfaceOld, and WTFOld.
4. Relaunch so the changes can take effect.
These are from Blizzard and after doing this my FPS was back up and I have not had a problem since.