One of the more difficult concepts to understand in professional soccer is the speed at which the game is played. Young players consistently point to the speed of the game as the aspect of pro soccer that is most difficult to adjust to as they make the leap to the top level.
It's important to be specific here -- MLS players are, overall, likely faster than those at lower levels of the game. But when you hear "speed of the game," you are hearing about how it feels to be out there trying to think when many of the other players on the field have been in the same situations so many times that they can operate by instinct.