We finish our normal runs in 17-19 minutes so with 40+ minutes on the timer every single time, pretty much always with 0 deaths (people make mistakes sometimesit happens) and we have averaged a 2.5-2.6% drop rate.
Others are claiming a 5%-10% drop rate.
Are they clearing it faster than us? because if not your theory already doesn't hold water.
If anything it seems like the slower you clear and the more mistakes you make the more DL you get because the only times we have ever gotten double DL were on runs where we wiped once, pretty sure our first double DL was actually on a run where we wiped on mistress and didn't even get the speed run chest.