With APFS, Time Machine underwent major change, to using snapshots to determine what to back up, rather than FSEvents. How well has this worked?
deep event scan
A post-mortem analysis on attempts to make my iMac dual-boot between Sierra and High Sierra. Can it be done in safety?
When Time Machine seems to take a long time preparing the next backup, or gets stuck, here is how to diagnose and deal with it.
The Eclectic Light Company 