DX11 was not intended to improve performance. Yes, you will see a frame rate decrease on hardware that was high end 3-5 years ago.
https://www.reddit.com/r/ffxiv/comme...ing_fps_gains/
I'm having the same crashing behavior on Win 10 with the latest nvidia driver on a GeForce GTX 660M card. The Media Feature Pack will not resolve this issue. That is for Win 10 installs that are missing features due to how you licensed your purchase.