I actually thought he played really well.
Used his speed, challenged defenders, played with a high compete level. But he's primarily a playmaker who relies on a lot of PP points and scoring off the rush - both things that tend to happen at a lower frequency in the playoffs because of how teams tighten up and the pace of play is faster. When everyone is playing at a playoff pace, there isn't usually a ton of chances to create off the rush. He may have to adapt his style a bit to really dominate in the post season - kind of like what we've seen from Crosby over time.
I noticed some similar "struggles" for McDavid at the WCH. As the pace picked up, he wasn't able to adapt his game - the competitive advantage that he gains from his speed and skill are less significant as the games get faster. At the WCH he often skated himself (and the puck) out of the play and was easily contained once he reached the boards. I think I saw him try the same "skate past the defense" move about a dozen times, and he was relieved of the puck every time before his linemates could get in tight enough to provide forecheck pressure, allowing for a quick one and done for the D.
It might do him some good to start slowing the play down a bit more - maybe instead of challenging the D off every rush, pull up, wait for some teammates, and try to establish some zone time. I don't think he's going to be able to rely on skating past defenders like he did in the regular season.
I wouldn't be shocked to see him "struggle" to produce in the playoffs until he learns to adapt his game a bit - find more soft spots in the defense, get his shot off a bit quicker, get to the net for garbage goals, and create off the cycle (obviously he's still very good at these things, but compared to his other skills, he could use some work).
None of that is meant as a criticism, but just opportunities for improving his effectiveness in the playoffs. For a 20 year old Captain in his first NHL game, I thought he looked really good and showed a lot of character.