Given that all three cameras have the same extra processor...???......it could well be down to mirror black out time!........
.....I paid some attention to the differences between the VF on the D850 with what seemed like a darker VF image, against the D5's more constant VF display.......certainly the VF in the D5 has very short lived blackouts.
That's not it. It might be it in some instances, but it's more than that.
I push the AF on my bodies with BIF under difficult lighting, low lighting, weather conditions, use TCs pushing up f stop, erratic birds. Sometimes more for the pushing of it to see what happens then to expect a great image.
In the worse of conditions the pro bodies D4, D4s, D5 will focus where the other bodies (D800/e, D810, D850, D750) will only hunt or fall flat on it's face. Doesn't matter that I can't explain why, all that matters is that's the way it is. On the other hand, a D5 is $4000 more than a D850 in Toronto, or near double. Not much you can buy at double the price that doesn't come with some benefits for the $$$ spent.
Am that's what I chaulk it up to. Designed as a sports and wildlife camera, and at twice the price. I don't need to rationalize it any more than that.