Am I completely humorless...

I noticed that too and thought it was odd.

I did notice that they stopped and huffed & puffed as the taxi got away.

I figured that they must conserve their energy. They do, after all, have a finite amount of that.

The Army major was trying to determine how long it takes for them to starve to death. As the jet plane flew over at the end, we saw that the Ragers were, indeed, starving to death.

So the Ragers do become more like cheetahs over time. They can only go all out for a short clip. That clip probably lasts longer right after infection (see how the soldier Mailer just keeps going at it without limit?), but as the time interval since infection increases, their energy reserves dwindle, and they can only step up to the challenge for short spurts.

I rather like the comparison to a mountain lion. Ragers are like instinctive animals just chilling for most of the day, but whenever someone catches their attention, it's as if that someone is interpreted as hostilely invading the animals' territory, and the animals attack! It isn't always about hunger or eating; it's about perceiving a threat and attempting to put that threat down.