This is just an idle observation over the last week or so of SSC and TK.
When you go into a fight with alot of things happening, large numbers of mobs hitting alot of people, or abilties that hit everyone at once. Pitbull chugs.
The best example I have for this is vashj, i was using Tapestry as a basis and removing some fo the healy mods to replace them with huntery mods. On Vashj i would get about 15fps with Pitbull, one try while everyone else was CRing i took out pitbul and put agUF back in. Instant increase in performance, i would get 35-40 during the same fight.
I disable almost all pitbull mods, i just keep, player, target, targets target, party, party pets, player pet.
No spark no interact lines, no estimated heals none of the extras, just the frames...no raid frames either.
Oddly I put ppitbull back in for Solarian on sunday night.

Thats my setup with a target frame in the middle of the two action bars. It did NOT exhibit the same problem as it did with Vashj.
It seems that with vashj the sheer number of things going on at once kills pitbull. I am not sure if pitbull processes combat events when you have combat text turned off, however I have no idea whats causing this huge slow down specifically for vashj.