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How to debug why LAN file copy speed is slow between 2 Apple machines on same LAN?

I’ve got two fast Apple machines on the same LAN (they are ethernet wired, plugged into the same switch, sitting next to each other).

The router is a UniFi Dream Machine Pro. Access to the outside internet is very fast.

But when copying a folder with a ton of files in it (about 500GB total), Finder copy estimates tens of days to complete, and Carbon Copy Cloner says 5 hours.

I’m using SSDs on both machines. This seems crazy slow to me (and I’m pretty sure I’ve done it before and it was much faster), how do I figure out what’s going on?