The only notification it gives for speed is how fast the dot is moving along the side of your head unit (on a garmin head unit, anyhow). I’ve found that to be enough (also, it lets me know if I’m being shadowed by a car because the dot(s) just stay in one place). The notification time has to do with the terrain as much as speed; on a straight road it can see a car that’s pretty far back.
As far as how it changes my riding, if I get an alert I’ll ride as far to the side as I reasonably can until I know the car is passed. I wouldn’t use it to move into the middle of the lane without also looking, though (that said, I can’t remember it giving a false negative notification, but I still look).