Slow progress of Time machine backup
I was having the same problem pretty regularly. I tried the usual tricks (stop backup, eject, mount, delete .inProgress files, stopping spotlight, changing the throttle priority (sudo sysctl debug.lowpri_throttle_enabled=0). Sometimes, this works. Other times it doesn't and it keeps crawling along at a few MB per hour.

One additional trick, that works more frequently, is to force quit the backupd process directly from the Disk tab of Activity Monitor. Then tell it to start backup. This usually works for me. Heads up: this may be a terrible idea in other ways (e.g. maybe it could damage existing backups). In my experience, it hasn't so far.