The ETA restic displays was based on a rate computed across the entire
backup operation. Often restic can progress at uneven rates. In the worst
case, restic progresses over most of the backup at a very high rate and
then finds new data to back up. The displayed ETA is then unrealistic and
never adapts.
Restic now estimates the transfer rate based on a sliding window, with the
goal of adapting to observed changes in rate. To avoid wild changes in the
estimate, several heuristics are used to keep the sliding window wide
enough to be relatively stable.
Michael Eischer