Lets suppose for a minute that I have a unit repswan ever 5 seconds. Will there be a difference in the rate it respawns if I use the countdown timer, and the game is fast - fastest.
Yes. The countdown timer matches the game speed.
Elapsed time starts from the very begining when you start the game. Countdown timmer can start when you want it ti start.
I'm pretty sure elapsed time works on map speed also. You ever play a game you knew took less time than it said? The time you're seeing is the elapsed time amount.