In this case transferring the data at the slow rate would have taken more than a week, so it's no small difference. Actually one side had a 10 Gbps line, so if the other side had had faster networking I could easily have exceeded the limit and gotten the transfer done more than 6x faster.

I used the term "1 Gbps line" just because it's a well known quantity - the limitation of Gigabit Ethernet. The point wasn't that multiplexing TCP can get you 6x better speeds, it's that it improved the speed so much that the TCP bandwidth-delay product was no longer the limiting factor in the transfer.