Nvidia cards use a system called CUDA to off-load processing from the CPU to the video card, CUDA is only available for Nvidia cards.

You could switch to using Apple's Final Cut Pro X, this is optimised for use with AMD video cards.The links the other replies refer to seem to be more about initial bugs between Adobe software and the then new 2013 model Mac Pro, the same links do not seem to be recent and therefore one could hope that the crashing and other errors - rather than speed issues have since been resolved. Therefore the performance is not as good as if you had Nvidia cards. Unfortunately in this case the Mac Pro comes only with AMD model video cards. The main issue between your model Mac Pro (which is actually still a 2013 model) and Adobe Premier is that Adobe optimise their software for Nvidia make video cards.