Beating PSG isn't easy; it's not just a matter of physical stamina

I was honestly quite surprised to see the players positioned exactly where they needed to be during transition sequences.
Because of that, the Barca players had to operate in very tight spaces,
whereas the PSG players were in their proper positions, allowing them to move the ball quickly once or twice.
This allowed their wingers to play in wide-open spaces, making the game look much easier—that must be the tactical advantage.
In hindsight, Arsenal's approach of playing defensively and not allowing transitions at all, similar to the Champions League final, seemed like a better way to handle it.