Tempo is a MF'er to deal with from a defensive standpoint. Can't sub. Hard to get calls into the defense.
Your defenders get gassed chasing the ball all over the place.
Imagine being a 250lb-300lb Defensive Lineman and you have to rush the passer then turn around a chase skill guys on the perimeter.(ex: bubble, screen, tunnel screen, hitch, etc) Remember: defensive coaches preach all-11 to the ball. After a couple plays those guys get frustrated and gassed. They start coming off the ball slower. That's when you run the football.
This is using the quick passing game to set up the running game.
This is what my coaching mentor (OC) used to do to my defense during practice scrimmages. If the offense completes a few passes my front-7 kids started to get frustrated and gassed.
That's why I 'SMH' when people say the spread ain't physical. Nonsense. It's physical as ****, just not in the way that we perceive as being physical. You're making those "physical" kids on defense (DE's, DT's, LB's) do stuff they don't wanna do...partake in a track meet. (they would rather bang in a phone booth)
The only downfall is if the opposing defense can get stops your defense is right back on the field. However, if that's happening frequently you can always slow your offense down a tad to give those kids a breather.