I was unsure about it as well. I have heard comments that lead me to believe speed can be measured as a one time (or nearly one time) deal.
The single 30.4 for Acuna isn't the same as the 30.4 sprint speed posted for Buxton though. It wasn't an apples to apples comparison. We have the apples (mostly) for Acuna now.
There is probably a graphic out there somewhere showing Buxton at 31+ for a trip from 1st to 3rd.