The DR exp bonus you get in a Low Level DR is based solely upon how far down someone gets sync'd, and has nothing to do with the time spent.
If you are doing the Low Level DR for experience, the further down you get sync'd the less you may desire a full clear, but bear in mind that you are still getting experience for each kill. It may not be the 3-5 k exp you can get per kill in Aurum Vale, but if you wanted that experience then you should have queue'd directly for it. The only time you should expect a speed run, rather than a full clear, in any sub-50 dungeon is when you form the party yourself beforehand. Expect Low Level DR to dump you in with someone who selected that dungeon specifically for current level experience.
