A GDC 2014 talk about OpenGL optimizations: NVIDIA, AMD, Intel Explain how to unlock 15X performance gains in OpenGL (another set of slides is also available here: How to Increase the Number of Draw Calls in OpenGL).
OpenGL: Approaching Zero Driver Overhead
http://www.pcper.com/reviews/Graphics-Cards/NVIDIA-Talks-DX12-DX11-Efficiency-Improvements
yeah, yeah, DX12 is “yet another” solution to make games more “multicore friendly” and to reduce overhead. I remember the first one from DX11 promo and the latter from DX10 promo. And guess what – it STILL DOESN’T WORK AS ADVERTISED. DX12 judging by slides will need yet another driver model which most probably will require yet another windows. Sorry but since OpenGL allows all that right now (and perhaps has from some time), doesn’t limit itself to x86 and Windows and does not generate additional costs (you want gaming rig – pay for DX inside Windows, you want new DX pay for new Win) so I guess it’s a KO.