I’ve often lamented the fact that jhbuild is dumb about the way that it does download, build, download, build, download, build. It totally misses the chance to download ahead of time so that it never has to stop building.
I just discovered “jhbuild update”, which will download everything ahead of time.
Of course, you can start “jhbuild update” and let it grab the first few packages and then “jhbuild build” in another terminal. As long as they don’t step on each others’ feet, that should speed things up a lot…