Major lag since Caves & Cliffs. 2 update

Since I updated my current Minecraft world to include the new Caves & Cliffs 2 update, my world has been experiencing major lag issues. The backup I created when I updated doesn't let me open it because whenever I try it just brings up a box telling me to update it, and asking me if I want to create another backup file. I tried changing video settings and uninstalling and reinstalling, all to no avail.