Yes, I am late to the party, but now that the Apple watch has completely destroyed the atmosphere at the Apple Regent Street store in London and left the place feeling a bit "tombish" I finally had a chance to actually spend some time playing around with the 5K iMac at the store. (usually there is somebody doing their facebook or whatever for a few hours and it's impossible to actually try it out)
The lag in mission control is shocking. I opened a few apps, moved them into different spaces and then flipped between them. I also hold shift when going into mission which slows the animations down so you can really see how smooth (or not) they are. I was shocked that the R9 M290X cannot handle basic animations and everything is very jumpy and laggy. Perhaps the display model has not been updated, but I hope that my experience does not match the real world, or that the M295X makes things a bit more bearable. My 2012 rMBP is much smoother when subjected to the same test and it is hardly cutting edge considering how old the internals are.
The lag in mission control is shocking. I opened a few apps, moved them into different spaces and then flipped between them. I also hold shift when going into mission which slows the animations down so you can really see how smooth (or not) they are. I was shocked that the R9 M290X cannot handle basic animations and everything is very jumpy and laggy. Perhaps the display model has not been updated, but I hope that my experience does not match the real world, or that the M295X makes things a bit more bearable. My 2012 rMBP is much smoother when subjected to the same test and it is hardly cutting edge considering how old the internals are.