Mac Thunderbolt 4 ports don’t support 10Gb/s transfers like USB 3.1 Gen 2

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A lot of users of M1 Macs have reported disappointing transfer speeds with external SSDs using Thunderbolt 4 ports, and tests indicate that most Thunderbolt ports on M1 Macs do not support USB 3.1 Gen 2 – which means they don’t offer the maximum 10Gbps transfer speed that would be expected from Thunderbolt 4.

A 2021 16-inch MacBook Pro M1 Max, and a 2022 Mac Studio M1 Max,  were used for the tests.


We have talked many times about the mess and the confusion caused by different USB-C devices, cables, and standards.

Also, there is a significant difference between the theoretical maximum speed and the actual maximum speed.

However, testing has shown that most Macs from Apple’s M1 series don’t support the USB 3.1 Gen 2 standard, making Thunderbolt transfers in theory and in practice slower than they should be.

A summary of the current speeds offered by different standards is offered by Eclectic Light.

  • From USB 3.0 onwards, SuperSpeed USB is supported With a speed of up to 5 Gbps.
  • Version 3.1 of USB 2 adds SuperSpeed+ at 10 Gbps with Version 3.1 of USB 2
  • USB 3.2 adds two lanes of SuperSpeed+ at 10 and 20 Gbps to USB 3.0.

Mac Thunderbolt 4 ports have been tested on the M1 Mac

Howard Oakley, the director of Eclectic Light, conducted a series of tests using two M1 Mac computers:

  • With a Mac Studio equipped with an M1 Max processor, 32 GB of memory, and a 256 GB internal SSD, all connected to an Apple Studio Display;
  • A MacBook Pro with a 16-inch screen, 2021, with an M1 Pro, 32GB of memory, a 2 TB internal SSD drive, and a screen that is integrated into the laptop.

There are two ways in which the speed capabilities of a data connection can be determined. The first is by seeing what the machine reports as its theoretical speeds. The first thing Oakley did was to test this on Intel Macs to validate that the SSDs and cables he tested were compatible with each other.

Each of them was connected to an Intel Mac (including an iMac Pro) and it was verified that they were able to establish a 10 Gb/s SuperSpeed+ connection with these Macs.

There were two types of cables used: a certified Thunderbolt 4 cable and the USB-C (data) cables that came with the cases. To verify that each was working, we established SuperSpeed+ 10 Gb/s connections to an Intel Mac.

In the System Information, the USB data for the SSD in terms of connection speeds are taken from the SSD’s entry within the USB data.

The second step is to measure real-life performance

As described in Stibium’s Help reference, I used my free app Stibium, version 1.0 (55), which wrote a total of 160 files that ranged in size from 2 MB to 2 GB to a folder on the SSD being tested, and read those same files back (Series Write and Series Read Test as described in Stibium’s Help reference). During the test, we followed the procedures outlined as part of the ‘Gold Standard’ procedure.

It was again Oakley who found that real-life speeds were lower with the newer Apple Macs, comparing Intel and M1.

Conclusions about the M1 Mac

This piece discusses the actual speeds achieved, pointing to the following observations. One of the most glaring of these problems is that the fastest storage devices perform at about half their expected speeds; this issue appears to be present in all M1 Macs. Connecting a Thunderbolt 4 cable to the front USB-C port on a Mac Studio Max results in speeds below 10% of what is expected; even the latest Mac Studio models don’t support USB 3.1 Gen 2; even the latest Mac Studio models don’t support USB 3.1 Gen 2?

  • At the moment, as far as I am aware, and since their release in November 2020, there appears to be no Thunderbolt port in any M1 model that fully supports Support for USB 3.1 Gen 2 with 10 Gb/s SuperSpeed+, or at least for SSDs.
  • There are currently only two ports in M1 Macs that seem to support USB 3.1 Gen 2 fully, those on the front of the Mac Studio Max.
  • There seems to be no problem with Thunderbolt ports on the M1 Macs being able to support 10 Gb/s SuperSpeed+ devices in a USB 3.2 port.
  • As far as SATA/USB-C storage is concerned, the impact on performance is limited, as the transfer speeds will be reduced from an expected 500 MB/s to 400 MB/s. There is little chance that this will have a significant impact on Time Machine backups stored on external storage, as backup I/O is throttled anyway.
  • The greatest impact is expected to be felt with external storage capable of transferring data at speeds approaching 10 Gbps, such as RAID arrays and NVMe SSDs connected via USB 3.1 Gen 2 rather than Thunderbolt 3. There is a high probability that expected transfer rates of 900 MB/s will be reduced to less than 500 MB/s in these cases, doubling the time needed to read a file.
  • Those who have USB 3.1 Gen 2 devices should connect them to a Thunderbolt 3 Dock or to the USB ports of a Studio Display, where they should perform better than using a USB hub.
  • Despite this shortcoming, all M1 Macs have been affected by this issue for over 18 months now. This bug should have been fixed over a year ago if it’s a bug in the firmware of either the Fabric core or the Fabric cores. It would have been smart for Apple to inform users and qualify its claim for the compatibility of these ports if it was a problem with the M1 series chips.
  • Apple must look into why using a Thunderbolt 4 cable to connect a USB-C drive to a front USB-C port on the Mac Studio Max results in unusably poor performance, fewer than 10% of the transfer rates expected.
  • In what has otherwise been a very successful transition to USB 3.1 Gen 2, one of the most shocking shortcomings is the lack of support for 10 Gb/s SuperSpeed+ in USB 3.1 Gen 2.

Apple has been contacted by the experts for comment, and we will update this post whenever we receive news related to this.

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