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For example, if I plug a USB 3.0 device and find that the I/O performance is rather slow, how do I check whether it's because the device is actually doing USB 2.0 (even though I'm using a USB 3.0 cable, connected to a USB 3.0 port on my MacBook Pro).

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Prior to Ventura, this was  menu > About this Mac > System Report...
Since Ventura, that's been made harder to find. It's now in System Settings > General > About, right at the bottom. About this Mac > More info will also take you there.

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Select USB on the left, then find your device in the list.

The reported speed ought to be a giveaway. Mine shows it's only running at USB2 speeds [but that's because I don't have a USB3 machine]

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List of speeds & common names…

USB 1.0 / Low-Speed: 1.5 Megabits per second (Mbps)
USB 1.1 / Full-Speed: 12 Mbps.
USB 2.0 / Hi-Speed: 480 Mbps.
USB 3.0 / SuperSpeed: 5 Gbps.
USB 3.1 / SuperSpeed: 10 Gbps.

Note that not everything needs to operate at the fastest speed. A magic keyboard connected to a modern M1 iMac shows just 12Mbps.

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  • mine says up to 12Mb/s, but that is listed under "USB 3.0 Bus", does that mean it is USB2? Apr 7, 2021 at 19:19
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    12Mbps is USB 1.1 - I've added a list of speeds to the answer.
    – Tetsujin
    Apr 8, 2021 at 6:29
  • In modern MacOS, System Report is only available inside System Settings. It's another level of depth from About This Mac.
    – Br.Bill
    Jan 14 at 1:22

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