Samba shuffles rather a lot of data, quite happily. You have not given us an exhaustive description of the shoddy wiring, dodgy switches and wonky configuration that makes up your network. If it was perfect, you would not be posting here.

The network is by no means ideal. I am transferring from a laptop on WiFi to a server on WiFi located some distance from the WAP. If I owned the place I would do a rewire, but for now it's the best I can do. I think I assumed that there would be error-checking involved when copying. Since following the advice here of using rsync i stead, I have found that files tend to fail in bunches and I need to rerun several times for it to actually complete. Am I right to assume that comes down to packet loss due to poor signal?

Your issue is probably hardware related. Test your network with say iperf3. Have a look at network stats. Don’t rely on cargo cult bollocks - do some investigations. Nowadays we have nearly all the tools as open source to do the entire job - we did not have that 30 years ago. Grab wireshark, nmap, mtr and the rest and get nerdy (or hire me to do it - don’t do that please!)

This is above my skill level for now, but I'm adding it to my notes to go back to. I have some ambition of upping my network knowledge in the coming year, and being able to do use such tools to troubleshoot would be great.