Men's Shed iMac decided today that it needed to do a full 83 GB Time Machine backup. After two hours, it was just over halfway through the operation & we had to shut up shop in another hour. So I stopped the backup, opened Terminal and entered sudo sysctl debug.lowprithrottleenabled=0 followed by the password. I then restarted the Time Machine backup.
That Terminal script changed the backup from a background operation to one with the highest priority. The remaining 39 GB backed up in about 15 minutes. Tomorrow another backup or two will be done and I'll have to repeat the same caper with Terminal.
There's a more complex process to make the change permanent, but frankly, this one's easy enough. Just do it before a backup in each login session.