You make some good points, and I thank you for taking your time to actually make them, as opposed to writing my suggestion off without explanation [eyes the second post].
I agree that humans should be slower than aliens, and I don't consider this making humans faster. If anything, the slide should begin at the current speed and slow to (nearly) a halt.
Right now (1.2), your chance of escaping through a vent is fairly low, as it takes quite a bit of time to get through the opening. You can get a little speed by hopping while crouched, but for the most part you will have lost energy sprinting toward the vent. If a human manages to get to a vent, I say we let him have it. It's far more annoying to die after reaching a perceived point of safety than to lose your target to size restrictions.
I also think that a slide could be predictable. Given that it should only be performable at a sprint (forgot to mention that XD), and while running forwards, it should be pretty obvious that a human sprinting toward a tyrant is looking to slide.
I have no intention of giving anybody an extra ability that provides a major advantage in certain situations. My aim is to provide something that may work if well timed, or the player gets lucky.
If at any point you get tired of going back and forth about a suggestion with me, just say so. I'll drop it. (And please point me back to this post if I don't :D)
And I was not joking about the tyrant slide. We should add a granger slide too. That would be awesome.