To be honest with you i haven't notice something like that.
But, depending on the situation. Are they faster even when they are red or or bw? Are the bots faster too? Have you seen only sertain players do that or you notice it by your self?
Sometimes players are using pills or adrenaline to by pass a horde. I do this frequently if i have points to waste. Exept if someone is doing Bunny Hop then good luck with that.
Note: Bunny Hop is not considering cheating when you aren't using any script. It's a glitch that allow the player to go faster while jumping. Check for HS Top on YouTube you'll get more info there.
Looks like the survivor settings are the Valve's default.
EDIT: No i haven't notice something.