Mantle talks started in early 2012 and actual development in 2013. I find it extremely unlikely that MS would require more time to finish up DX12 than AMD Mantle.
MS didn't have DX12 ready for the XBOX ONE launch which is super weird. Why wouldn't they release it for the XBOX ONE first and then later modify it for PC if needed (is there even going to be a difference)? Vulkan, which is estimated to be finished around August, is going to be just a few month completed after DX 12 (about a year since development started).
That speed at which MS works makes it very hard to believe that actual work on the API started before Mantle especially considering that in the same time span they finished Mantle and then ported it to SPIR-V and made it work on none GCN hardware (Valve demo running on Intel GPU at GDC 2015).
But hey, maybe you are right and MS is just that slow. No wonder they didn't manage to implement proper MT into their API for about a decade of multy core CPU's.
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