As btrbk diff already delivers a super nice overview of the difference of a parent and child snapshot, one could also get a time estimate during a btrbk run. This could be done by extending the existing --progress option, or by introducing a --progress-total.
Choosing this option, for every newly created snapshot btrbk diff would be run before btrfs send/receive such that the pv command can get proper -s option for the progress update.

This way it is easier to estimate the total time left, especially in cases where the network bandwidth is the limiting factor.