USB 3.1 Gen 1 Transfer Speed Through Hub
The hub to upstream computer will be capped at 5 Gbps. You can do 5 Gbps to one of the two devices at a time, or 2.5 Gbps to both devices simultaneously.

If you want to move data from one downstream hard drive (A) to another downstream hard drive (B), what you're really doing behind the scenes is reading data from hard drive A into host computer RAM, then writing from RAM to hard drive B. I don't think you can write directly from Hard Drive A to Hard Drive B with just a USB hub; you need some processor in the chain to tell the data where to go. 

This will limit you to 2.5G Gbps reads from Hard Drive A and 2.5 Gbps writes to Hard Drive B, because the hub's max data rate is 5 Gbps total.

I don't think you can speed this up without using a second USB host controller on your upstream host computer. If Hard Drive A and Hard Drive B are both connected to a laptop directly, over two separate USB 3.1 hosts, then you could hit 5 Gbps reads from A and writes to B at the same time. The USB hub does create a bottleneck in your data rate.