Well, it's not that a six legged beastie can't be fast - it's that you can't make them run faster than a four legged beastie of the same size and weight, because putting some of your running power into another set of limbs doesn't make you any faster, and arguably slows you down because of aerodynamics and other factors. It's a matter of what's the most efficient conversion of biological strength into forward motion. And any adaptation that this hypothetical six legged beastie could have that offsets the disadvantages of extra limbs can be evolved by its predators as well and used to better effect.
I can't get past that. Just a personal flaw, I guess. I can't make myself think that way. It's probably because I have a degree in earth science - I've studied evolution at a greater depth than most people, and knowing what I know about it, I can't design a creature that's supposed to be functional and non-magical without it being influenced by that. This is just me though. More power to you if you can do it.
I think Edward has the right idea. If straight-line speed doesn't make sense, then put the creature into another environment entirely. Another possibility would be, say, having the creature move through the treetops, where having two extra limbs does translate into better forward motion and easier navigation of the terrain.
I've actually got the Art of Avatar book, and it's interesting to see how they handled the hexapod designs - essentially, they couldn't make the six-legged gait work, so they hacked it into a pseudo-four legged gait by having the front two pairs of limbs working as one. Yeah, the Avatar beasties are fast, but they're essentially four-legged beasties with one set of limbs duplicated.
So I guess it's the difference between designing something that could exist but doesn't, or something that just plain shouldn't exist at all.