What is really important is the force and consistency of the force necessary to push a key, and the way it pushes back. They keys have to slide, without friction, which excludes most cheap keyboards which have a high friction mechanism for positioning each key. Yes, in most keyboards you can't check that until you try them. And usually low profile keyboards are better at providing a faster typing rate because they lack that mechanism, if we're talking low cost. Such low profile keys rely on rubber (or whatever) to hold the keys in position, and use classic plastic nest only for securing less common movements. Logitech UltraX keyboard has a different plastic mechanism which offers less friction and better stability.
Too low profile is also bad, when it comes to flattening the keys. Some laptops today provide a totally flat keyboard, which doesn't offer much tactile feedback. It's harder to feel the keys shape and position, thus it is necessary for each key to have relief (high edges, lowered center).
When you talk about very fast typing, faster than 6 characters per second, you might want to take a look at mechanical keys offered in high end keyboards such as Das Keyboard.
So what you need is:
- Less friction
- Key relief
- Stability (stable key position, no wobbling)
- *Might also take a look at keyboards that claim to be ergonomic. They usually offer more comfort, and you may type faster on them.