Both DX and OpenGL work with the hardware abstraction that does not exist any more. As a result, the API calls cannot be trivially converted into the hardware c

Both DX and OpenGL work with the hardware abstraction that does not exist any more. As a result, the API calls cannot be trivially converted into the hardware commands. Most importantly - there are multiple paths the translation can follow and each will produce very different results from the performance point of view. There is a reason modern video card drivers are hundreds megabytes big - they contain app-specific optimized code for hundreds of games and applications. With the above said, 20% difference is solidly in the area of driver versions differences.