Baseball Savant's bat tracking page is fascinating.
Fastest bat speed in the game? Giancarlo Stanton. Not surprising.
Longest swing in the game? Javier Baez. Also....not that surprising.
Having a lot of bat speed doesn't make you a great hitter (Stanton) but it certainly helps.
Having a short stroke might sound great, but it doesn't mean you're hitting the ball hard enough to be a major threat (Arraez).
As with most things, the ideal is somewhere in the middle.
Also, context does matter. Aaron Judge has one of the longer swings in baseball, but he's also 6'7.
There's probably less of a reason for Michael Harris II to have a long swing, though.