I just read the review on this lens from Photographylife. See https://photographylife.com/reviews/nikon-z-28-400mm-f-4-8-vr/2
The reviewer does not think the lens focuses significantly slower, at least not slow enough to be an issue.
Here's what he does say: That said, despite the speed, it wasn’t quite as precise as a wide-aperture lens would have been. I blame this on the greater depth of field exhibited by an f/8 lens; the camera just has a harder time placing it perfectly. Sometimes, it would exhibit a little bit of front-focus or back-focus on a moving subject, especially at 400mm (and worse in low light). It was never terrible, but if you’re a dedicated wildlife photographer, most other 400mm lenses will have a bit of an advantage here.
I noticed that focus accuracy when tracking a flying bird with my Z8 and 500mm PF F/5.6 is better than when I have the TC-14E III mounted on the lens. With the TC mounted the effective aperture is also F/8.
The reviewer does not think the lens focuses significantly slower, at least not slow enough to be an issue.
Here's what he does say: That said, despite the speed, it wasn’t quite as precise as a wide-aperture lens would have been. I blame this on the greater depth of field exhibited by an f/8 lens; the camera just has a harder time placing it perfectly. Sometimes, it would exhibit a little bit of front-focus or back-focus on a moving subject, especially at 400mm (and worse in low light). It was never terrible, but if you’re a dedicated wildlife photographer, most other 400mm lenses will have a bit of an advantage here.
I noticed that focus accuracy when tracking a flying bird with my Z8 and 500mm PF F/5.6 is better than when I have the TC-14E III mounted on the lens. With the TC mounted the effective aperture is also F/8.