In the TBT3-UDV specifications, it lists the USB ports as such:
Does this listing correspond to the number of underlying USB controllers in the dock driving the ports (with the ports in each line sharing the bandwidth of the controller/USB speed), or is it just an inventory of ports (that might have a different configuration of underlying USB controllers/shared bandwidth) and if so, what is the actual underlying configuration?
E.g., if a user plugged in 4 different USB-to-SATA SSD drives (each capable of 5 Gbps/~500 MB/s throughput) into the USB-A ports in the rear, would the drives in aggregate be capable of 20 Gbps/~2000 MB/s throughput (that, up to the PCIe x4 limit of the Thunderbolt uplink)? Or maybe just an aggregate 10 Gbps/~1000 MB/s, if each set of two rear USB-A ports have their own independent 5 Gbps USB controller (for 10 Gbps throughput combined), or maybe just 5 Gbps total, if all the ports actually share the same 5 Gbps controller under the hood?
Likewise for the front 5 Gbps USB-A port. Does it share bandwidth with any of the rear ports, or does it have an independent 5 Gbps capability
In terms of the performance of connected USB devices, to the best of my knowledge we have not tested the specific scenario you describe so I am afraid we cannot make any definitive statements.
That said, ultimately the limiting factors will be the capabilities of each USB 3.0 host controller and the total available bandwidth over the Thunderbolt connection to the host which is allocated dynamically.