Mac mini Thunderbolt Bridge slow transfers with MacBook Air

Just as a test, test file transfer speed between the two devices using Target Drive Mode.

Once you connect the Target Drive device, look in System Information and determine the type of connection made, USB or Thunderbolt and the connection speed. Then do a large file transfer, 1GB or more. What transfer speed do you get?

This could identify if the cause is is a basic file system issue or hardware issue.

If this works well, then there would seem that there may be a network configuration issue when the Thunderbolt connection is used in a network mode on the MacBook Air side (since an i5 MacBook Pro was fine).

Your iperf3 result proves that your hardware, ports, and Thunderbolt cable are fully capable and running at high speeds (approx. 37 Gbps). The drop to 40 Mbps during file transfers or disk speed tests happens because macOS is routing your file sharing traffic through a slow fallback network interface (like a lingering Wi-Fi Bonjour/IPv6 link-local route) instead of your explicit Thunderbolt Bridge IP. 

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac? If so uninstall them completely and retest.

Make sure to disable or bypass iCloud Private Relay because it might slow upload speeds or prevent logging in to some sites.

For example, with Private Relay ON, Ookla Speedtest reports 530/300 Mbps but if I choose Safari > View > Reload and Show IP Address before doing the test it reports 530/530 Mbps for my 500 fibre via Ethernet.

Use iCloud Private Relay on Mac - Apple Support

pscheepe wrote:

Mac mini Thunderbolt Bridge not working properly

I have a MacBook Air M4 and a Mac mini M1 both running Tahoe 26.4.1 connected via thunderbolt bridge and when I tested the connection speed with iperf3 I get 37GBits/se but when I transfer a 1 GB file or test with Black Magic disk speed the internal drive in the other computer I only get 40 MBits/sec.

FYI, whenever I see 40MB/s for drive speeds, it tells me the connection is limited to USB2 data transfer rates, or the drive in question is an old Hard Drive where 40MB/s transfers are the average.....the latter does not appear to apply in this case with two M-series Macs. Definitely try @woodmeister50's suggestion and the Apple System Profiler.

I also wonder if BlackMagic's Disk Speed Test could be utilizing a different interface connection from your other methods or is iPerf being used during BlackMagic's Disk Speed test?