This may well not work for everyone but heres is what happened for me the last few days.
Started playing Thieves Guild update on Sunday just gone and hearing about all the issues I thought perhaps I should update my GPU driver to the latest. Did this before I updated. Well it seems this is what has caused all the issues.
I would get 8-10fps on login on both 32bit and 64bit clients. Rather strangely if I alt-tabbed out of game and back in the fps drop would resolve itself and then return everytime i zoned ingame.
I just reverted to an older driver about 20mins ago after having all the same issues 40mins ago and all is resolved no FPS drop no pink horses.

Here are the details
GPU = nvidia GTX680
New driver used that exhibits issues = 364.51
Old driver used with no issues = 358.91
Other guild m8's I talked to about this said they were fine on the new driver, so if this works for you or not may well depend on your current GPU model.
Hope this helps some of you!
Edited by Alusi on March 15, 2016 7:49PM