Despite having a gigabit hardwired ethernet connection to my Macbook Air, when I have Time Machine backing up to an iMac also hardwired on the same switch in the same network, the transfer speed is often less then 1MB per second.
If/when I copy a file from the Air (Catalina 10.15.7) to my iMac (Mojave 10.14.6), I get a speed of about 65MB per second. As it is, it takes about a month to get the 1Tb disk in my Air backed up, all that time sitting there with a wired connection and doing little else. If I could get 65MB/sec with Time Machine, it would take less than 5 hours to backup that 1Tb disk.
The backup disk is a 5Tb portable USB drive connected to the server (my iMac running Mojave) and configured to "share as a Time Machine backup destination". Several Macs (High Sierra, Mojave, Catalina) are successfully (but slowly) backing up to it over WiFi.
How can I get the initial backup to go as fast as possible?