There was nothing erratic about Lafreniere's first 3 years. He was drafted on to a team with a strong top 6 so was slotted on the third line playing with fellow rookies and youngsters. Each season, in limited time and almost non existent pp time, Lafreniere put up more points than the year previously. The only change that happened (beyond continued player development) last year is he went from 3rd line to 1st line. The production increased accordingly. The PP time did not change.
The only real difference between these two is Slaf jumped up to 17 minutes and top 6 in year 2 because there was no one in his way. Laf didn't get there until year 4. Top teams with Stanley Cup aspirations have slightly different deployment strategies than bottom feeders.
I'd also point out that in year 2 Lafreniere, in less minutes (4 whole minutes less per game!) and significantly less pp time, scored 19 goals to Slaf's 20. It's not Laf's fault that Kakko was a black hole and couldn't score if his life depended on it.
Even Strength year 2: Lafreniere: 19g, 10a, 29 pts Slafkovsky: 14g, 22a, 36pts
Now look who each played with during their times at even strength and try and tell me that Slafkovsky is much further along (other than in his organization's depth chart) than Lafreniere was at the same time?