Direct X CPU instruction that were handled by the CPU is now handled by the GPU! https://www.youtube.com/watch?v=RE2hNrq1Zxs&feature=youtu.be...
The switch to DX12 is just that you can write the same sort of code on Windows and Xbone (the 360 had enough extensions that you sort of had to have separate rendering backends for 360 and Windows, despite them both potentially being DirectX 9).