Comentario oficial
The extra area being covered shouldn't slow the speed down. It may have made you travel over a darker area on the approach which will affect the speed of the whole mission. The flight speed for the mission is determined by the first 70% of the approach leg.
You can't edit it mission because it is tied to any flights that have already been done on it but you can hit the Unlock button in the top right which will make it editable again so you can save it with different parameters.