I too am having some performance issues with the latest update. All AJAX activities are lagged, tremendously. I let it run at one point to load the media embedding forms and i lost the ability to interact with firefox, at any level – it froze, not even the loading bar changed – for nearly 12 minutes. I updated yesterday evening and that is when all of this occured.
I will be down grading to the flat 2.5 version of the app until some sort of fix is found.
