The Canon 70-200 is equally fast on a 1Dx2 or 5D4, but sluggish on the A7r2, and even on the A9.
No it isn't.
The new E FL ED is faster than Nikon's G VR II (Canon's equivalent), with better VR, and better imagery. Why would you think Canon's elder lens would be "as fast" as Nikon's newest?
Further, Canon's AF performance on a Sony, with an adapter, might not have the same translation as Nikkor's AF (E) on a Sony.
At the same time, the Sony 70-200 is just as fast and accurate on the A9 as the Canon is on a 1Dx2 or the Nikon is on the D5. Trouble is, it just isn't sharp, except when stopped down two stops.
Exactly. Fast AF + poor performance = you get lousy results "quickly"
Wouldn't it be better to get
awesome results at a decent pace?
You're not going to get equally-fast AF with an adapted lens, nor will you have the invaluable eye AF. You don't buy a top-tier lens and body only to get 'average' AF.
On the contrary, many top-tier lenses have average (even zero) AF.
I guess my real point is
try it out for yourself and see ... rather than base your belief system on "reviews" and assumptions.
Weren't all of the Sony 70-200 GM reviews "glowing" ... and yet
your own experience was mediocre? Take a lesson from that.
Point being, you were prejudiced
in favor of the Sony ... but it disappointed ... so consider the possi-(probi)bility that you might be prejudiced
against the Nikkor ... and yet y
ou might be thrilled in real life.
With a 30-day trial/return with most reputable dealers, what have you got to lose?