Omnirom on the p6810 was the fastest of the Marshmallow builds. You might want to give it a try, along with screen casting. Casting did work, but because it was software encoding on the p6810, it was a bit slow and would sometimes crash out.
If a Nougat Rom ever gets made, it should be a bit smoother on our old devices. At this point i think development has stopped on our old devices. I have contemplated trying to make a Nougat build, but i have a Nvidia Shield tablet now, and Nvidia do a good job of maintaining the device as compared with Samsung, so i mainly use my old Samsung as a book reader these days
so i will try omnirom marshmallow