It works sometimes, but too often I'll load a video from a channel I watch frequently, and it forgets my speed setting. And if you skip far ahead in a video, so YouTube has to reload the stream or change the quality level, the speed adjustment is consistently lost.

the sound is terrible in slomo mode

doesn't remember setting for the next tab so effectivity is limited. I'd have to change the speed manually at which point i could just use the native speed control.