At the end of the Combustion mission when you're supposed to ride the elevator all the way up you can kill yourself once the door has closed and the elevator has begun its ascent. Doing this will spawn you at the very top long before the elevator reaches and you can save a good chunk. The best (and perhaps only) way to do this is to use a grenade launcher called Stay Away and Warlock Solar Grenades. I'm not sure if this works solo (it should, it's not a darkness zone) but it does with a duo or fireteam

Another little trick, at the very end of the Hope mission you can end the mission without having to wait for the door to open and reaching the trigger. Once you've deployed your ghost, run to the area beneath where you spawn when you normally load into Siren's Watch (or have a partner wait there if you're doing duo/fireteam). Then stand on the generator closest to the edge. From there you can jump for the platform. Aim for where it meets the wall on the left. Climbing onto the platform will trigger the cutscene. In a duo or fireteam run, this saves a good few seconds. Solo it is rather risky and may not save as much because you need to run back over