He doesn't play full speed, doesn't he? He plays that way using change of speed because of his knees. Of course he is good bball player and we're lucky to have him.
If you have played basketball on any level after injury knees you will understand that better.
Lmao what are you on? If you have significant knee issues that prevent from playing "at full speed" you definitely aren't going to be operating your game from a change of speed mentality as that's going to put a lot of pressure on your knees as much, if not more, than just playing a consistent higher speed.
The likelihood is that Butler has some kind of degenerative knee issue from ignoring/playing through knee pain. Those things are tough to treats because you have to find the root cause, which might not always be the knee itself, then figure out how to treat it over time while playing through it. In some cases you might have to stop your sport altogether and do a long rehab process, bit hopefully that's not the case.