The new script is out (Ubooquity 1.8.0).
It preloads 2 pages ahead of the one you're reading.

Let me know if it works better than before.

James, Justin, I'm marking this problem as "solved" as you can now explicitely choose the directory where the new theme is created (using the "workdir" option from Ubooquity 1.8.0). But let me know if you still encounter problems with this.