Thunderbolt ports aren’t all the same
USB-C ports that support Thunderbolt 3 differ between models, and those on the same Mac may not all be the same. This article explains what each can and can’t do.
USB-C+Thunderbolt 3 ports on Intel Macs are run by Intel chips that don’t support USB4 connections, while those on all Apple silicon Macs rely on Apple’s own hardware inside the M-series chip, and have full USB4 support. Connect most USB4 SSDs to an Apple silicon Mac and they’re significantly faster than Thunderbolt 3, but move the same drive to an Intel Mac and it will perform at less than a third of that speed. That’s because they then fall back to USB 3.2 Gen 2 speed of around 1 GB/s. The only way around that may be to use a dock, but that’s an expensive solution.
Intel chips in Intel Macs have one controller for each pair of USB-C+Thunderbolt 3 ports, so you shouldn’t expect full Thunderbolt 3 performance on both ports simultaneously. Apple silicon Macs do have one controller for each port, and should be capable of handling all their ports at full speed throughout, without any interactions between them.
