Slow/laggy rMBP (mid 2014) - nVidia card not being used?
You could try installing the NVIDIA Web Driver for your GPU. That's what I did to try and improve GPU performance on my old MBP with GeForce 320M. It also adds a preference pane where you can always switch back to using Apple's default GPU driver.

Since the NVIDIA driver archive is a pain to navigate I recommend using the NVIDIA Web Driver Auto Installer from here to automatically find and install the correct driver for your GPU.