Offizieller Kommentar
If the light conditions were not able to support the estimated flight speed the aircraft was likely slowed down to accommodate this. You can turn this off in the settings.
https://support.dronesmadeeasy.com/hc/en-us/articles/209866886-Light-Based-Speed-Adjustment
If it was terrain aware it will also take longer and that is not accounted for in the estimated time.