After quite a bit of testing, I have concluded that the USB bus on at least my Mac mini M1 does not actually support "USB 3.1 Gen 2 (up to 10Gb/s)" as listed on the Tech Specs page.
I have a Samsung T5 USB-C SSD. It supports "USB 3.1 Gen 2 10Gb/s" (Gawd, I hate USB 3.1+ naming conventions...†)
When I plug it directly to one of my 16" MacBook Pro's ports via USB-C-to-USB-C cable, it connects at 10 Gb/s
When I connect it to my new M1 mini, on the other hand.... If I go through my Thunderbolt 3 Dock, it connects at the full 10 Gb/s:
But if I connect directly to the other Thunderbolt 3/USB-C port, it only connects at 5 Gb/s, yes, using the same USB-C-to-USB-C cable as both previous connections:
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