Have You Played... F.E.A.R.?
Is this your first encounter?
F.E.A.R. is a scary first-person shooter, in which you're part of a tactical unit dispatched to deal with paranormal activity. In this case, it is a ghostly girl called Alma who has a habit of suddenly appearing in dimly lit corridors.
But to uncover the truth behind this spooky kid, you have to contend with a large number of cloned supersoldiers. Luckily enough, you happen to a particularly athletic supersoldier yourself. One that's capable of moving so blisteringly fast, it's as if time slows around you.
And F.E.A.R.'s slow-mo effect is ridiculously fun to use. Pair it with the nailgun and you've got yourself a devilish combination. It's sort of sickeningly delightful, the way you can observe a soldier tumbling in the air with a nail through his skull, before they're pinned to the wall with a snap.
I like F.E.A.R.'s clean aesthetic too. Reminds me a bit of Half Life, with its industrial grey environments and straight lines.
Writing all this just makes me want to play it again, to be honest.