Thanks for that.
Sadly, I've tried and I've built another 5 Keystones at that point, and I'm still getting there with 25.35 seconds left, plus or minus 0.2 seconds, so the Keystone is just not cutting travel time even by 1 second. Even if I throw every rounder I have at it, I don't have enough time to do the next steps, so I'm officially stuck. I suppose I could wipe the save data and go back to the beginning, and see if that makes a difference, but... I've rebuilt my rounders so many times, I'm just... yeah. But thank you for a great game, good luck with the progress, maybe I'll come back to it some time.