A lot of rubbish floating around on the web and youtube. Hardly any useful info can be found.
As already experienced with the Z6 and Z7, the AF-S lenses perform very good, and on the Z9 even better. Also thanks to the improved Auto-Focus. Even the 300/2.8 AF-I works fast on the Z9
Auto focus accuracy is better than with the F-mount camera's. The Z-mount sensor, in combination with the FTZ adapter, works smoothly. Working just as fast as 'native' lenses.