I don't think what you describe has anything to do with the CPU. If the CPU isn't able to run the game logic at 60 fps no matter how much you lower the resolution, the game won't hit 60. And if dropping the resolution gets you 60 fps then the bottleneck was clearly the GPU, not the CPU. In the case of Uncharted 4 the game seems to be bottlenecked by both the CPU and the GPU, since even at 540p the game still has a ton of drops. Or maybe the system just lacks the memory bandwidth necessary for 1080@60 fps and that's why dropping the rez gets you much closer to 60 fps, but in any case it can't be the CPU.