ZestyShorter round timers force the attacking team to commit more to their pushes as the timer ticks down. A lot of people worry that this will lead to turtling teams doing nothing and playing for the round reset more. While this is true it means failed pushes where a repush isn't possible (most of the attacking team gets out) lead to a round reset rather than another stalemate.
The problem is that the shorter it is set the more linear it makes the game. If it's short enough to have an impact then it essentially reduces the game to win mid > push last with almost no prospect of variation from this formula. There is no real point playing 5cp in this case apart from the lack of maps for other modes, you've effectively reduced it to a weird version of koth.