Hi,
Yes, the USB ports on my host machine are USB3.2 capable. However, the carrierboard that I am using for flashing is only USB2 capable on the flashing port.
480Mbps is the theoretically limit without USB protocol overhead + network stack over RNDIS, etc. The 250 Mbps actual data transfer per USB Port is totally realistic when considering this. USB can not be the bottleneck because I am not even able to reach the 250 Mbps when using NFS during initrd flash.
I also don’t expect much improvement by switching to USB 3.0, as WayneWWW already mentioned.