Apple - Time Machine backup is slow. How can I speed it up?

On El Capitan to High Sierra OS - the io is throttled in some cases. You can review these steps in case they apply in your case. Not everyone sees this speed up, but it shouldn’t be hard to test and reverse if it doesn’t help you. Anyone on Mojave and newer should look to other fixes like testing a backup to a blank or erased drive.

Time Machine ridiculously slow after El Capitan upgrade

The throttle should be evident in observing the backupd usage of the filesystem:

sudo fs_usage backupd

Writing a command to /etc/sysctl.conf and then restarting should disable that throttle:

echo 'debug.lowpri_throttle_enabled=0' | sudo tee -a /etc/sysctl.conf

Or without restarting:

sudo sysctl debug.lowpri_throttle_enabled=0

You'll want to watch for other performance issues (extra CPU usage, other IO slowing, increased energy usage when backing up) after making that low level system tuning adjustment.

Unfortunately, the solution in this case was:

Format disk (lose old backups)

Now the disk backs up at full speed.