Why is File sharing so slow on TB bridge between M1-M4 Macs?

Disk speeds presume certain file sizes, and certain file caching present.

If you are backing up to iCloud or Time Machine, that will slow speed.

If you are sharing the thunderbolt bus (the controller on the computer) with other devices, that will slow down your Mac to the slowest component on the bus.

If you have a Mac that is underpowered for the bus, that can slow you down as well. This is especially true of notebook computers. Have an AC/DC powered hub can often optimize speeds.

If either drive is over 85% full, swap file build up can slow down transfers.

The transfer of many small files, under the block size will be much slower than the transfer of lots of large files, because each action of read/write slows down the transfers.

You'll notice what the true block size is by making a small TextEdit saved text file. Actual text files are often a byte per letter. The size of those files will indicate the block size, which is the minimum size that must transfer at once to have better transfer times.

Data transfer of system files that are in use when doing cloning will be slower, as the Mac has to determine what state the file is in to ensure only a good copy of the file gets copied over.

Energy Saver Power Settings for put hard drive to sleep will slow the bus down even further until you restart the computer.

FYI, you can have USB3 cables with USB-C connectors which do not support Thunderbolt protocols & transfers. Plus some USB cables with USB-C connectors may not transfer data at all & just provide power to charge devices, while others like the Apple USB-C cable that ships with some Macs is only rated for USB 2 transfers which max out at 45MB/s.

Anything related to USB-C connectors & ports is extremely confusing because manufacturers are not always clear on what each connector, port, or device can do. Many product manufacturers (even some respected brands) exacerbate the situation by providing extremely misleading product information meant to trick people into thinking their product is the best/fastest, but if people were to look extremely closely at the technical specifications, data sheets, and footnotes, they may discover the truth....or they may discover nothing which means the manufacturer is overselling their product & tricking the consumers.

USB-C is just a connector standard & has nothing to do with data transfers.

Most Thunderbolt rated cables will have a lightning bolt icon on their USB-C connectors.

If a cable or device has no markings to indicate their specific function & protocol supported, then assume the worst....it will at best support some form of USB transfers at some unknown speed.....at worst, it is just for supplying power.

Good quality cables with USB-C connectors that only support the USB transfer protocols should have clear markings on exactly what USB protocol is supported.

Seeing transfers at 30MB/s makes me think something in your setup is causing the devices to only operate at USB2 speeds. Make sure the devices are connected directly to the computer and disconnect all other external devices in case one of them is causing a problem. Avoid any adapters, hubs, or docks as they may not work as you expect. Also try using different USB-C ports on your computers in case there may be a hardware issue with one of the ports.

Do you have any other USB3 or Thunderbolt 3 devices you can use to test the cable and each Mac to confirm proper functionality?

Edit: Also, are you sure you have selected the proper destination volume for the Black Magic Disk Speed Test?

It is important that the location selected has the necessary permissions for you to write test files for the speed test and that volume/folder is not being synced to the cloud or shared with some other service which could impact the results of the test.

Along with the great suggestions that others have provided, make sure that you are not overlooking anything obvious. Are you connecting to the Thunderbolt to Thunderbolt ports on your Macs and not the USB 3/4 ports (also USB-C connector). A question worth asking is also if you have any antivirus or security software installed that may be trying to inspect traffic or files?