That works out at 1.8 GB/s upstream from the Mac, and 2.4 GB/s downstream to it, totalling 4.2 GB/s. In my latest tests, when the fastest of three SSDs was reading, and the slower two SSDs were writing simultaneously, read speed was around 1.8 GB/s, and the two SSDs wrote at a steady 1.2 GB/s each. If those might discourage you from considering a hub, its performance during combined reading and writing appears to exceed what you’d expect through its single Thunderbolt connection with the Mac. That’s particularly significant for those connecting both displays and SSDs to their hubs, although my previous measurements with an Apple Studio Display showed little reduction in SSD benchmarks. Controllers give priority to the needs of video over data, so driving a large display will result in poorer SSD performance. There’s also the problem of using DisplayPort video over Thunderbolt, in combination with data devices like SSDs. As each lane delivers up to 10 Gb/s, those SSDs we thought complied with a 40 Gb/s standard were actually only halfway there with 20 Gb/s. That’s because Thunderbolt 3 includes ‘Low Power’ controllers (notably Intel’s JHL6240) that only use two of the four PCIe 3.0 lanes available.

By now we should be used to Thunderbolt not quite delivering what we expect: the best examples are all those certified SSDs that can’t read or write any faster than 1.5 GB/s. The answer could lie anywhere from macOS to Thunderbolt controllers in hubs and SSDs. Even when writing to that SSD alone the Mac only managed 1.5 GB/s, the same as many older and cheaper Thunderbolt 3 SSDs. In two cases, when an SSD normally capable of writing at 2.2 GB/s was connected through a hub and written to at the same time as another faster SSD, its write speed collapsed to 800 MB/s, the same as an SSD of half the cost. Last week I started to focus on the last of those, just how well can Thunderbolt 4 hubs work with some premium high-speed SSDs? The answer isn’t as clear as I had hoped, and gets murkier by the day. Docks are expensive, bulky and inflexible: give me a compact hub and a couple of cheap adaptors to chain other devices from it, and I’ve got more flexibility, lower cost and better performance. Just as with USB-A in the past, no sooner are you marvelling at how many ports your Mac has got, than you’re wondering how to connect all your peripherals. I seldom stick my neck out and make predictions, but if I was looking to sell any Mac peripheral it would be a Thunderbolt 4 hub.

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