I've used a 200-500 on a D500 and D810. I've struggled in low light with the D810, but that may not be a surprise. I found focus speed to be acceptable with both when at distance, but it's certainly not as fast as, say, a 70-200 f/2.8, and certainly not my 200 f/2. I don't think the body made a vast difference to the speed - I suspect it's cranking the AF elements over quite some distance.
