É uma velocidade de barramento, você pode confirmar lendo o arquivo "speed".
cs-server@:~/Downloads/logs$ cat /sys/bus/usb/devices/usb1/speed
480
cs-server@:~/Downloads/logs$
cs-server@:~/Downloads/logs$
cs-server@:~/Downloads/logs$ lsusb -t
/: Bus 02.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/10p, 5000M
/: Bus 01.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/16p, 480M
|__ Port 6: Dev 2, If 0, Class=Human Interface Device, Driver=usbhid, 1.5M
|__ Port 8: Dev 3, If 0, Class=Vendor Specific Class, Driver=ftdi_sio, 12M
cs-server@:~/Downloads/logs$