Jah is not slow, he's not a gazelle like Noel, but he has very quick feet, schooled both Porzingis and Towns one on one because of his quickness. That can translate to defense, but it'll take a couple years, heck, it'll take Simmons a year or two to learn to play NBA defense. Offensively skilled players often are protected on defense in college b/c it's too much trouble to coach up a one and out, just keep 'em out of foul trouble.
If Jah was 23 with 4 years of NBA experience, yeah, what you see is pretty much what you'll probably get, though many big men mature later and peak in the their mid-20s. But we're talking a 20 year old rookie, a bit early to assume he can't improve.