Well, they said Windows 8 was better than Win 7 and had pre-release benchmarks to prove it showing a 1-2% advantage . It failed to live up to that premise and in the end, Win7 proved to eb faster than Win 8.
Win 10 showed the same 1-2% in pre-release versions using today's popular games. and DX12 is supposed to bring increased performance once game devs start coding for it. However, we saw wonderful results telling us how great Mantle was going to be with specific benchmarks created that were tailored to it that showed big gains. However, these advantages never materialized in actual games released. So will DX12 deliver ? Right now all we have is speculation; given past hoopla, the next big thing rarely lives up to expectations...