That isn't the reasoning at all. The NFL doesn't have guaranteed contracts, so in a very real sense the players in the NFL are playing more to stay in the NFL than players in college are playing to get into the NFL. Also some of the highest paced teams in college are the teams full of players with no hope of playing in the NFL.

The reason some college teams play faster than the NFL (and it isn't all teams) is because there is a wider talent distribution in college. It is possible to win games there by being faster and playing faster than your opponent. Talent is more evenly distributed in the NFL. You can just out athleticism your opponent.

Another factor is that both players and coaches spend more time strategizing in the NFL. It is harder to just run the same scheme every game without it being eventually exposed.

Strangely enough, this sport has had a similar problem with playing for time that afflicts football/soccer. It used to be a horrible problem that almost destroyed the sport until the invention of the shot clock. I think it may have gone a bit too far in the other direction, to the point where you can ignore everything but the last five minutes of most games, but football/soccer could really use something like a shot clock to force offensive action.

And time for advertisements.

The biggest sporting event in the US sport calendar year is known - not for the crunching tackles, not for the high flying antics, not for the pin in the needle plays...no sir - it is known for ads. Superbowl ads are bigger than the sport. When I was younger I though Superbowl meant ad marathons.