Here is a really good [article][1] on this subject:

> When a track is designed for Olympic competition, the designers are
> going to make the track as “fast” as possible — which usually means
> that the track will provide a great deal of grip, but will be as hard
> as possible to allow the runners, especially sprinters, get the best
> push-off from the track.
> 
> Now there’s one other trade-off that needs to be made. Sprinters
> typically want the track as hard as possible, while distance runners
> would like to see a bit softer surface. Sprinters are only on the
> track for a matter of seconds in their races, so they would prefer a
> very hard surface to get the most speed out of the track. Distance
> runners are racing on the track longer and would prefer more cushion.
> When the designers sit down to create the running surface, they need
> to decide if they’re going to make a preference toward sprinters or
> distance runners.
> 
> Anecdotally speaking, I’ve heard that some Olympic tracks — like
> Atlanta’s– were designed more with sprinters in mind so while many
> sprint records were set there, distance runners felt a little beat-up
> after racing on it. In Barcelona, I heard the opposite, that the
> distance runners really liked the track, but the sprinters didn’t feel
> it was fast enough.
> 
> So the Bird’s Next in Beijing? If you’re hearing that the track is
> “very fast”, you can probably read that to mean “very hard” — and very
> favorable to sprinters. As we see lots of world records fall, like
> Usain Bolt’s world record in the 100M, it is probably an indication of
> a great surface for sprinters.



  [1]: https://coachjoeenglish.wordpress.com/2008/08/19/track-and-field-what-makes-a-track-faster-or-slow/