Outdated addons. That is the most likely your problem. Since you mentioned you started playing again, you probably didn't update them? Try turning them all off and see if it's better. If it is, download new ones.
Also, did you OC your CPU?
I've got 2500k @ 4.3 GHz and 560 Ti. On highest settings, on Ultraxion I get between 50-60 fps WHILE frapsing, and I get around 40-50 in Orgrimmar at peak hours.