Why is a Time Machine backup so slow over a fast network?
Time Machine is really poor at working over a network, and I’ve stopped doing this. The best way to run Time Machine now (macOS 11) is to give it a dedicated encrypted disk (which TM will reformat to APFS). The backups will be very fast and you will be hard pressed to notice them, but even backing up to an HFS+ local drive will be orders of magnitude faster than over your LAN.
(The reasons for this are somewhat opaque, but the backup process is very different when backing up over a network as your target is not a disk or even a network folder, but a sparse image file on the network and this makes, as you’ve seen, a huge difference. The behavior seems to have gotten worse in recent versions to the point where it’s just not worth it.)
Another option you might consider is to use a syncing tool to sync your data to a location on the target machine that is part of its regular backup. I’d do both, but then I’m a &quot;three backups are the minimal starter set” sort of guy.