Recovery, both physically and mentally.
When you see teams sluggish on the tail end of back-to-backs, it has little to do with a plane ride, and more to do with having played the night before.
Sure both teams will have to deal with it (unlike the regular season), but tired teams often play a more defensive style, knowing they don't have their legs.
Also the B's newfound lack of goaltending depth could hurt them. Like starting pitchers, goalies are not used to playing consecutive days.
The NHL's RTP has been great so far, but this is something easily avoidable.