P: Performance problems iMac Retina 5k

I've been using the new imac retina, and i have enormous problems with the performance of Photoshop. If i for example use the selective color correction, this does not run smoothly. Other functions also work delayed. I noticed if i do not use Photoshop in full screen mode, everything goes back to normal. A friend used the predecessor model imac without 5k resolution, but has no problems. I suspect Photoshop has problems with the resolution of 5k. I hope Adobe fix this problems because Photoshop works slow and delayed, the performance is terrible. I bought the retina iMac 5k, since my previous mac is getting old and slow. But now is it so, that my new retina iMac, run equal slowly as my old iMac. Adobe please fix this problems, so i cannot work.