He's not adjusting to the pace of the game because its too fast, hes adjusting to going from playing an 'in your face' style of play to playing a more measured, trap-style system. He was playing a very specific style for the last two years, working on the skills he needs to be a successful NHL player. In Finland, him using those skills that made him effective arent really useful right now. If he battles in the corner, he's going to be out of position. If he closes the gap, he's left himself out of position, etc etc. He's looking quite good while playing, 90% of his shifts are fantastic but he makes the occasional misplay that would be something that works in the NHL but doesnt in Finland.
You point out him having a disappointing regular season last year... he was 19. There were only 9 players in the league from his draft or the draft after that have become NHL regulars. He wasn't struggling in juniors at 19, he was struggling at the NHL level. Saying he was injured last season which contributed to his struggles isnt a variable as much as it is an explanation because he had a full NHL season where he contributed as an effective 3C and then a playoffs series where he was again effective as a 3C.
I have a hard time believing that him as 20 year old wont develop from beyond his current role. No I dont think he will become Joe Thorton but low end 1C isnt beyond him at all. He may likely end up as a 2C, cool. You cant base his potential off a 30 game stretch thats surrounded by 2 good stretches of game. He looked great at 18, he struggled at 19. Only 6 players from his draft class including him have played more than half an NHL season.
Evander Kane, Chara, Duchene, Hornqvist, McDonagh, Simmonds... off the top of my head, those are all guys who played in Europe during the lockout and struggled. Heck Kane got 2 points in 12 games or something like that playing KHL (Minsk I believe). All of these guys were (or became) top 6 forwards or top pairing dmen in the NHL. Lots of players in the lockout sturggled to adapt to the leagues they were playing, some popped right in and produced but others took a while to get warmed up. This mindset that NHL talent will immediately become game-breakers in Europe was disproved during the lockout. For every Malkin there was a Kane during the lockout, dont judge a player by production, judge him by the skills he displays. Its bad scouting to look at the numbers and not the whys of the numbers