If you thought infantry was useless and slow, you obviously didn't play the game long enough to become good at it.
I was consistently in the top 3 out of 16-32 on my team, I'd say I was pretty good.
However, if you didn't have a vehicle you weren't going to capture a flag, takes too long to get too and fro. And if a vehicle is coming at you and you're not in one you're a bit screwed, unless you're either anti-tank, or he doesn't see you and you get your C4 on him.
Compared to vehicles infantry was slow and useless. Best way to play is hop in a buggy, go to a flag, hop out, capture while picking off anyone spawning, hop in the buggy, go to the next flag, have another squad defend the one you just captured, repeat.