I think the baseline for RS intensity was pretty high in the O6, especially compared to the 70s and 80s. There were no multi-year contracts, no players association, players had little power compared to coaches and owners, and there was lots of talent in the minors. Players from that era have said they had to play hard every game or they would lose their job.
That era was more matchup dependent than anything. Some players could do well in a certain matchup and struggle in others. Take Andy Bathgate. He was a superstar regular season scorer with the Rangers. And he was a good playoff scorer against other American teams. But the majority of his playoff games were against dynasty Montreal and Toronto teams, and he scored less than half a point per game in those games. He just wasn't good enough to be a superstar at that level of competition.
Bathgate's playoff scoring
Vs 50s Habs: 10 GP, 5 P
Vs 60s Leafs: 6 GP, 3 P
Vs 60s Habs: 19 GP, 9 P
Total vs dynasties: 35 GP, 8 G, 17 P
Vs BOS/CHI/DET: 19 GP, 13 G, 18 P