Maybe not so much a CPU bottleneck, but rather an API bottleneck.
Microsoft might be one of the biggest software companies in the world, but their software really sucks.
UWP sucks, the Windows Store sucks, Windows is full of bugs, DX12 is slow, and the Xbox App is a piece of crap almost of the level of Games for Windows Live.
Youre very right, but DX12 can be great (if used well, look at shadow of the tomb raider with different apis for example) and has many features, its just that they can totally botch it as we have seen with so many unreal engine 4 releases. DX12 should allow for better cpu utilization than 11.
Greater potential, but also far greater potential for disaster. The fact that unreal sucks so much at utilizing multiple threads so that we cant brute force it and IPC being the main thing that we can see performance increases in regards to cpu performance sucks, but thats probably on Epic.
In the end its the devs resposibility to get their software to run properly on platforms where they are selling it, otherwise they shouldnt expect to make money.
Its possible to make unreal engine 4 game runs well as we have seen before, but its very easy to fuck it up which is what ends up happening most of the time.