He says that "Pinboard has more than 350K commits and is 20GB in size when cloned fully." I'm not clear though, exactly what "cloned fully" means in context of the unoptimized/optimized situation.

He says it went from 40 minutes to 30 seconds. Does this mean they found a way to grab the whole 20GB repo in 30 seconds? seems pretty darn fast to grab 20GB, but maybe on fast internal networks?

Or maybe they meant that it was 20GB if you grabbed all of the many thousands of garbage branches, when Jenkins really only needed to test "master", and finding a solution that allowed them to only grab what they needed made things faster.

I'm also curious about the incremental vs "cloning fully" aspect of it. Does each run of Jenkins clone the repo from scratch or does it incrementally pull into a directory where it has been cloned before? I could see how in a cloning-from-scratch situation the burden of cloning every branch that ever existed would be large, whereas incrementally I would think it wouldn't matter that much.