I haven't been following him that much before, but he's a big kid and very heavy. It usually takes some time for these big and heavy forward kids to get their full potential out and adjust their skating and movement to the NHL level.
Rantanen, Barkov, Wheeler to give you some examples. And especially if Slaf had some crazy growth during short period of time.
I'd say at this point he's similar prospect to Rantanen, but then again he's one of the best wingers in the league currently so it's unfair to expect that Slaf would get there, but it's possible.