Oh, sorry, I didn't read the title well enough.
Nonetheless, I don't think one can reliably conclude the read out speed for stills from the one for video. At lower bit depths, read out is sped up. As video often uses 12, 10 or even 8 bit it is at an advantage compared to stills.
Also, stills will read the full sensor height and therefore take more time, as they do not use the 16:9 format.
And with different cameras, the specific modes matter. I have not been able to find a good source for the bit depths of the Nikon Z9 formats. But if they were to be 14 bit instead of the 12 bit of the R5 for example, that should be factored into a comparison, not just the pure speed.
Do you mind sharing where you got those numbers? I find very few sites actually test the read out speeds at all and tests for stills are even more rare.
This German site has good numbers on the read out speed in milli seconds for video with the R5 and A1, but those show how close they actually are and do not reflect the numbers you quote (for stills?) :
| 8K 30p | 4k | crop |
R5 | 16 | 16 @ HQ, 10 @ 60p, 8 @ 120p | / |
A1 | 15.5 | 8.1 @ 60p, 6.2 @ 120p | 11.3 @ 24p, 10.5 @ 60p |
Sources:
R5 Test Slashcam
A1 Test slashcam