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I've read that in many cases, the motherboard basically acts as a USB hub. So if you have two 10gbps USB ports, and you try to transfer files over both, you won't go over 10gbps between both of them.

Is there a way to determine which of these ports share bandwidth?

lsusb -t:

/:  Bus 06.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/4p, 10000M
    |__ Port 3: Dev 2, If 0, Class=Mass Storage, Driver=usb-storage, 5000M
/:  Bus 05.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/4p, 480M
/:  Bus 04.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/4p, 10000M
/:  Bus 03.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/6p, 480M
    |__ Port 6: Dev 2, If 0, Class=Hub, Driver=hub/4p, 480M
        |__ Port 3: Dev 3, If 0, Class=Human Interface Device, Driver=usbhid, 1.5M
        |__ Port 3: Dev 3, If 1, Class=Human Interface Device, Driver=usbhid, 1.5M
        |__ Port 4: Dev 4, If 0, Class=Human Interface Device, Driver=usbhid, 1.5M
/:  Bus 02.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/4p, 10000M
/:  Bus 01.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/6p, 480M
    |__ Port 1: Dev 2, If 0, Class=Human Interface Device, Driver=usbhid, 12M
    |__ Port 2: Dev 5, If 4, Class=Mass Storage, Driver=usb-storage, 480M
    |__ Port 2: Dev 5, If 2, Class=Audio, Driver=snd-usb-audio, 480M
    |__ Port 2: Dev 5, If 0, Class=Audio, Driver=snd-usb-audio, 480M
    |__ Port 2: Dev 5, If 3, Class=Vendor Specific Class, Driver=, 480M
    |__ Port 2: Dev 5, If 1, Class=Audio, Driver=snd-usb-audio, 480M
    |__ Port 6: Dev 3, If 0, Class=Hub, Driver=hub/4p, 480M

I have 10 ports on the back (4x 2.0, 2x 3.0, 3x 3.1A, 1x 3.1C), at least two on the front. But there are only six "buses". Is a "bus" basically a USB hub, where putting devices on different buses would increase simultaneous bandwidth?

Interestingly, only three of the buses say 10000M, where there are actually four 3.1 ports. Does plugging in a slower device on the same port slow things down?

The audio interface is connected to the only USB C port, but that bus is showing as 480M. Unless the onboard audio controller is considered to be "plugged in" to the 2.0 USB bus? Only the keyboard and mouse are plugged into ports labeled 2.0.

How do I determine if (and which) USB ports share bandwidth?

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Is a "bus" basically a USB hub, where putting devices on different buses would increase simultaneous bandwidth?

Most likely.

Does plugging in a slower device on the same port slow things down?

It mustn't: https://superuser.com/questions/477241/does-a-usb-2-0-device-connected-to-a-usb-3-0-hub-slow-down-the-hub-to-2-0-speeds

Unless the onboard audio controller is considered to be "plugged in" to the 2.0 USB bus? Only the keyboard and mouse are plugged into ports labeled 2.0.

It might be wired to a USB2.0 bus. Audio doesn't really need a lot of bandwidth. 192KHz 32bit audio requires just 11Mbit/sec bandwidth which is well below roughly 320MBit/sec offered by USB 2.0.

How do I determine if (and which) USB ports share bandwidth?

USB ports which are connected to the same bus share bandwidth. Also, a USB controller is normally connected to your CPU via a PCI-E/DMI bus which has its own maximum bandwidth: https://linustechtips.com/main/topic/823767-does-usb-31-use-pcie-lanes-why-there-is-no-fully-31-motherboards/?do=findComment&comment=10318581

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