I think there are a number of issues here.
DX12 does not automatically bring better performance. My understanding is that DX12/Vulkan performance is now almost entirely up to developer optimization, and GPU vendors cannot optimize for it as they once did. Overall that should benefit AMD, but puts NVIDIA at a disadvantage, since NVIDIA often have big performance improvements in their GameReady drivers for DX11 games.
As I understand it, there are two main things that DX12 brings for performance: much better multi-threaded support, and asynchronous shaders. Though it was far more limited than DX12, NVIDIA already support multi-threading in DX11 while AMD are single-threaded - so AMD have the most to gain here. NVIDIA do not support asynchronous shaders on their current hardware so they do not get that performance improvement.
People are looking at average framerates instead of minimum framerates.
Here's a 1440p benchmark from someone who was complaining about DX12 performance on a system with a 390X and a 6600K:
So DX12 performed worse, right?
Wrong. Look at the minimum framerates there. It jumped up from 10 FPS on the Syria and Geothermal Valley tests to 30 FPS!
Now admittedly their maximum framerates did drop quite a bit, so there is probably room for optimization - or perhaps DX12 is also rendering additional effects - but I would say that going from a 10 FPS minimum to 30 FPS is significant.
Minimum framerate is the most important number for performance by far, not maximum/average framerate.
But that said, I would much rather see frame-time graphs than min/max/avg framerate numbers. Those numbers don't really tell us much about how things are actually performing.
