Massively speed up Time Machine backups

This seems like a bad idea...

You almost assuredly do want to use low priority, throttled I/O for time machine backups. (see IOPOL_THROTTLE) Why? Because it prevents backups running in the background from interfering with foreground applications.

Honestly, if you're interacting with an app in the foreground then you almost certainly don't care how long the backup is going to take, and you almost certainly do care if the backup makes your app slower. And if you walk away from the computer then the backup's I/O will run unimpeded and unthrottled, i.e., fast.

Most apps don't use a high queue depth and so waste a lot of SSD performance. With SSD a QD up to about 8 is free.