I have been having this awful problem with choppy frame rates in World of Warcraft instances. Not anywhere else, just instances.
After removing all of my interface add-ons, I noticed the choppiness was gone. Adding them back in one at a time narrowed it down to one called “Gatherer“. That one tracks potential herb and mining resource spots for me. I never really relied on it so I took it off for good. I have read the newer versions don’t have the problem, but I really don’t care at this point. I’m just happy I can go into an instance again and move freely. Woohoo!