[2019] Why do survivors fast-vault at different speed?

It's based on the angle of approach, and you'll also need a run-up. If you sprint at a window head on, you will do a fast vault and go straight through the window. But if you sprint at it from the side or bump into the window before vaulting, you'll do a medium vault.

To get the fast vault, just be sure to run straight at the window from about a meter or so away and press space before you touch the wall. Otherwise, you'll get the medium vault. This isn't always possible, which is why some loops are less effective one direction than in the other. Take the ironworks, for example. If vaulting in, you do not have the space required to do the fast vault in, so doing the loop that way means you'll gain less ground. It also means that as a killer, you'll want to try to force them to change directions if possible.