Josi has definitely lost a step. Last time I remember him really having the juice to get back and break stuff up from out of position, which used to be a hallmark of his game, was the 21-22 near Norris year. Still a great player, just having to adjust to time like everyone else.
But yeah, top line speed isn't everything. I think most people would agree McDavid is the best skater in the league, and while he's very fast, I think last I saw he was outside the top ten in the league in flat line speed according to the edge numbers. It's his ridiculous ability to change speed and direction more than anything.