It appears that watching cinematics contributes to mission time, which should not be the case. I just completed a mission in 7:30, then replayed without watching the cut scenes and finished in 3:15. I didn't really do anything else different, the mission was basically driving from A to B.
I am replaying missions to get gold, but I would also like to watch cut scenes. This should be fixed. You can probably get gold in most missions and watch cut scenes, but knowing this I am now skipping them so i don't come up just short and miss the time limit, requiring another replay (which is annoying in itself since you have to reload the 'main' world, then reload the mission again).