Xcode 9 GM has severe graphics issues running iOS11, if you run iOS10 on an older device or in the simulator it seems to work fine. This is due to the new A11 Bionic chip having a new GPU architecture which of course means Apple had to rewrite all the graphics drivers which their still in the process of doing. The present graphics performance looks like the CPU is doing all of this work since it pegs out at 100% in the simulator, and Apple hasn't rewritten the low level graphics routines in iOS11 to take proper advantage of the GPU on the MacBook yet. Apple has stated they are aware of this issue, and will fix it. I would expect a new Xcode 9 release in the next couple of weeks that fixes most of these issues since it also has to be impacting Apple's developers also.