macOS Sequoia 15.3 has improved Thunderbolt 5 performance

There have been many reports of problems with Thunderbolt 5 support in Apple’s latest MacBook Pro and Mac mini models with M4 Pro and Max chips. Among the more concerning have been poor performance when accessing SSDs through TB5 docks and hubs, and the inability to drive more than two 4K displays through those. This article looks at what has changed, and what can currently be achieved when accessing SSDs either directly or via TB5 docks or hubs.

When I last tested these, using a Mac mini M4 Pro and Sequoia 15.2, I found that speeds measured through a TB5 dock were generally at least as good as those through a TB4 hub, with three notable exceptions:

Write speed from a TB5 port to a TB3 SSD through a TB5 dock fell to 0.42 GB/s, little more than 10% of that of a direct connection and similar to that expected from a SATA SSD operating over USB 3.2 Gen 2.
Write speed from a TB5 port to a USB4 SSD through a TB5 dock fell to 2.3 GB/s, about 62% of that expected.
Write speeds to a TB3 SSD through a TB5 dock occur at about half the expected speed, just as those through a TB4 hub.

Methods

Three Macs were used for testing:

iMac Pro (Intel, T2 chip) with macOS 15.1.1, over a Thunderbolt 3 port without USB4 support.
MacBook Pro (M3 Pro) with macOS 15.2, over a Thunderbolt 4/USB4 port.
Mac mini (M4 Pro) with macOS 15.3, over a Thunderbolt 5 port.

The results for the first two are taken from my previous tests, and here used for comparison.

The dock used was the Kensington SD5000T5 Thunderbolt 5 Triple Docking Station, with a total of three downstream TB5 ports. I’m very grateful to Sven who has provided his results from an OWC TB5 hub to support those from the dock.

Other methods are the same as those described previously. The TB5 SSD tested is one of the three currently available or on pre-order from OWC, Sabrent and LaCie (no, I’m not going to tell you which, as I’m still in the process of reviewing it).

Single SSDs

Results obtained from measuring read and write speeds on a single SSD at a time are summarised in the table below. Those that are concerning are set in bold italics.

Performance of the TB5 SSD when connected direct or through the dock was highest of all, and around 150% of the speeds achieved by the next fastest, the USB4 SSD, and around 180-250% those of the TB3 SSD, the slowest. Direct connection of USB4 SSDs to the TB5 port in macOS 15.3 resulted in even faster speeds than a TB4/USB4 connection using 15.2. Thus, a TB5 port with 15.3 delivers best performance over all types of external SSD tested here.

Of the three exceptionally poor results seen previously:

Write speed from a TB5 port to a TB3 SSD through a TB5 dock improved greatly from 0.42 GB/s to 1.6 GB/s, the same as in other Macs.
Write speed from a TB5 port to a USB4 SSD through a TB5 dock improved from 2.3 GB/s to 3.8 GB/s, the same as when connected direct.
Write speeds to a TB3 SSD through a TB5 dock remained at 1.6 GB/s, about half the expected speed, just as those through a TB4 hub.

This anomalous behaviour when writing to a TB3 SSD through a TB5 dock was also found by Sven in his tests on the OWC TB5 hub, and seems common to most if not all TB4 and TB5 docks and hubs. I haven’t seen any explanation as to why it occurs so widely.

Paired SSDs

Encouraged by these substantial improvements with Sequoia 15.3, I measured simultaneous read and write speeds to a pair of USB4 SSDs connected to the Kensington TB5 dock. Stibium has a GUI so can’t perform this in perfect synchrony. However, it reads or writes a total of 160 files in 53 GB during each of these tests, and outlying measurements are discounted using the following robust statistical techniques:

a 20% trimmed mean, giving the 20th and 80th centiles;
Theil-Sen regression;
linear regression through all measured values, returning a rate and latency.

Measured transfer rates in each of the two USB4 SSDs are given in the table below.

The first row of results gives the two write speeds measured simultaneously when both the SSDs were writing, similarly the second gives the two read speeds for simultaneous reading, and the bottom line shows speeds when one SSD was writing and the other reading at the same time.

When both SSDs were transferring their data in the same direction, individual speeds were about 3.1 GB/s, but when the directions of transfer were mixed, with one reading and the other writing, their speeds were similar to a single USB4 SSD. Total transfer speed was thus about 6.2 GB/s when in the same direction, but 7.2 GB/s when in opposite directions.

Multiple displays

Many of those buying into TB5 are doing so early because of its promised support for multiple displays. I haven’t yet seen sufficient evidence to decide whether this has improved with Sequoia 15.3. However, OWC has qualified full display support of its TB5 hub as requiring “native Thunderbolt 5 display or other displays that support USB-C connections and DisplayPort 2.1”. One likely reason for multiple displays not achieving support expected, such as three 4K at 144 Hz, is that they don’t support DisplayPort 2.1.

Which macOS?

As the evidence here suggests, macOS 15.3 or later is required for full TB5 performance, and OWC now includes that in the specifications for its TB5 hub. It also states that TB3 support requires macOS 15, although USB4 should still be supported in macOS 14 Sonoma.

Recommendations

TB5 SSDs are faster than USB4, which are faster than TB3, in almost every combination. The only exception to this is a USB4 SSD connected direct to a TB3 port, which is likely to be limited to 1.0 GB/s in both directions.
When pricing allows, prefer purchasing a ready-made TB5 SSD. If it’s to be used with an Intel Mac, confirm that it there supports TB3.
Self-assembly TB5 enclosures remain expensive at present, and a USB4 enclosure may then prove better value, provided that it won’t be used with an Intel Mac.
Avoid writing to a TB3 SSD connected to a dock or hub, as its speed is likely to be limited to 1.6 GB/s.
Ensure Macs with TB5 ports are updated to Sequoia 15.3 or later.
Ensure Macs to be used with TB5 docks or hubs are updated to Sequoia 15 or later, or they may not fully support TB3.