I died at level 13 for the first time, then I went to my level 56 toon and crafted the best stuff a level 13 or lower could have and that helps kill quicker. I didn't have to wait till I found the yellows, I just crafted them.
The thing I'm noticing though is that while I'm gaining many more levels/map, it takes so much longer to kill things, that the net gain is very little. If you play on normal, you'll only be level 5 when you hit the crypts but it takes about 40 minutes to get there. On master, I'm level 13 when I hit the crypts but it has taken me almost 4 hours to get here. In both cases, I clear all the black as I travel. On normal after 4 hours of play, I'd be at least level 13, maybe higher.