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Tests show Thunderbolt ports on M1 Macs don’t fully support USB 3.1 Gen 2 speeds

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Howard Oakley of The Eclectic Light Company has been doing some testing of the Thunderbolt ports on M1 Macs, and it seems that real-world performance doesn’t match Apple’s claims. When using USB 3.1 Gen 2 devices with M1 Mac Thunderbolt ports, performance doesn’t meet the expected connection speeds of up to 10Gb/s, meaning real-world read/write speeds will also fall short of expectations.

Before diving into Oakley’s test results, it’s important to understand the USB 3.x standards. As outlined by Oakley, USB 3.0 supports SuperSpeed USB at 5Gb/s. USB 3.1 Gen 2 includes SuperSpeed+, which doubles the possible speeds to 10Gb/s. USB 3.2 allows for two-lane SuperSpeed+, doubling the maximum possible speed yet again to 20Gb/s.

Apple’s M1 Macs are equipped with Thunderbolt 4, which supports speeds up to 40Gb/s using Thunderbolt 4-equipped storage and supports USB 4, which is also up to 40Gb/s. While Apple doesn’t explicitly mention USB 3.1 Gen 2 on its list of tech specs for M1 Macs, users reasonably expect that Thunderbolt 4 ports that support USB 4 would deliver the (lesser) speeds of USB 3.1 Gen 2. However, users have found that their M1 Macs don’t meet this expectation.

The M1-equipped MacBook Pro (14- and 16-inch models) have three Thunderbolt 4 ports. These ports support speeds up to 40Gb/s, so why are USB 3.1 Gen 2 devices not delivering the expected speeds?

Oakley’s results confirm the underwhelming performance. He performed testing with an Apple Mac Studio with an M1 Max chip and a 16-inch MacBook Pro with an M1 Pro chip. In both cases, the computers ran macOS 12.3.1 in Full Security Mode with no extensions enabled that might affect transfer speeds. To measure transfer rates, Oakley used his app, Stibium. When using Thunderbolt 4 ports on an M1 Mac with certified USB-C and Thunderbolt 4 cables, the connection speed was 5Gb/s. However, when using USB-C ports, like the pair of ports on the front of the M1 Max Mac Studio, the connection speed was 10 Gb/s. Further, when connected to an Intel Mac using a USB-C cable, the connection speed was 10Gb/s.

What does all this mean? USB 3.1 Gen 2 storage connected to an M1 Mac via a Thunderbolt 4 port delivered 5Gb/s connection speeds. When connected to a USB-C port, like on the front of the new Mac Studio Max desktop computer, the connection speed was 10Gb/s. USB 3.2 storage, which can operate at up to a 20Gb/s connection speed, was 10Gb/s when connected to a Thunderbolt port on an M1 Mac. Given the halved connection speed, if you are using USB 3.1 Gen 2 storage, read/write speeds will correspondingly be about half of what users expect when using a Thunderbolt port on an M1 Mac.

Oakley concludes that if the shortcoming is a bug, ‘it should have been fixed over a year ago.’ If it’s not a bug, but is a defect of the M1 chip family itself, ‘then Apple should have informed users and qualified its claim for the compatibility of these ports.’ 9to5Mac has reached out to Apple for comment. It will be interesting to see what Apple says if they respond. For the full breakdown of Oakley’s results, click here.


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