He is referring to input lag: the delay from input to visible change on the monitor. The double buffering and compositor step added by Wayland adds some delay.

You may not personally notice it, or be used to it, but it's always there. Some people are more bothered by input lag than others, and there are so many compounding factors today that make it worse than ever. It's not only the double buffering in graphics subsystems like Wayland. Monitors are also often adding a frame or two delay to bump up their pixel "response times" and reduce ghosting by looking at future frames.

I swear, using Wayland or MacOS or Windows 10 on the fastest hardware today still feels slower and laggier than 10 year old hardware running plain X11 or Windows 7 (with the compositor turned off).

Given the choice between tearing and input lag, I'll take tearing any day.

It seems your subjective feeling is not reflected in measurements. At least for kwin wayland is as fast as uncomposited x11

https://zamundaaa.github.io/wayland/2021/12/14/about-gaming-...

Don't these results show a 20% slower based on the median?

Plus, the test uses a display at 120Hz, which is a substantial head start -- by making each frame of delay half as long compared to us mere mortals running at 60Hz.