Large fps drops in some areas (9.0+)

[R_OReilly]

I noticed this during Brewfest and now again during Pilgrim's Bounty. My fps drops from 60 to single digits when I cross the Gates of Orgrimmar and into Durotar (entire zone). Northern Barrens and Azshara are normal 60 fps but Ashenvale drops to 15 fps. I've done this with Carbonite Maps as the only enabled addon with the same results. Disabled tracking for herbs/minerals/pets to no effect. Has anyone else experienced this behavior?