Mac M2 Pro Thunderbolt transfer rate slows after connecting 2nd and third external monitor

You have M2 but here is info about Mac mini 2018, if it helps any:

If you connect displays to the Thunderbolt 3 ports on your Mac mini, use different Thunderbolt buses when possible. Don't connect more than two displays to any bus. If you connect a 5K or MST 4K display, you need to use a different bus for each of these displays (Bus 0 is besides the Ethernet port and Bus 1 is besides the HDMI port):

Try staggering the Displays and Drives across the four Thunderbolt 4/USB-C ports to see if that helps.

Drive > Display > Drive > Display or optionally Display > Drive > Display > Drive

If I remember correctly, the 4 Thunderbolt ports are split between two Thunderbolt busses. I would try connecting the displays to work off one bus then put the SSDs on the other bus.

However, it is unknown how the Thunderbolt controller in the Mini manages the busses. It is possible that the entire Thunderbolt bandwidth is shared between the two busses, in which case, all that data is being passed through basically a single pipe that has two spigots. So, if that is the case, data transfer could slow down because the displays are automatically assigned a certain amount of bandwidth and the SSD drives get what ever is left.

This is only a guess at what may be causing the issue and if any one else has a more definitive knowledge of the Mini Pro's Thunderbolt controller can correct me if that is not the case.

Actually I had these kind of issues when transferring files from my old MacBook to my new one.

Turned out that Apple's SMB implementation is extremely slow when transferring multiple small files. I had to transfer 93k+ objects, yet only 5 GB. Estimation after starting was way over one hour.

Compressing the files into one archive: 3.5 min, transfer a few seconds, decompressing on the target machine less than 2 min.

You're welcome.

How and what are you moving?

It is normal for large continuous data transfers to slow down in comparison to short smaller transfers.

Is the SSD enclosure plugged directly into the Mac mini, not one of the USB-C ports on the display? The display ports are not Thunderbolt, so will be slower.