In my experience from converting a large project from pro to sdk, there is NOTHING thats SLOWER in sdk. Some of the performance INCREASES you may come across are laughable at best, but like I said, ive never seen anything slow down.
So having said that, im guessing it has to be something different in your code.
*edit well wait a second. U6 got rid of a lot of extra junk, thus resulting in a overall fps increase for most people (my project jumped from 55 to 65ish fps). Sdk is still basically 5.8, so this could be the reason for the difference.
All you need is zeal