That does seem a long time for a single pork butt. I usually cook two at a time, and it takes anywhere from 12.5 to 17 hours.
I recently smoked a single 8.8 pounder. Lit 30 briquettes for Minion start with H2O in pan and set bottom vents at about 25%. I don't fuss with measuring grate temps anymore, but the unreliable Weber dome gauge said it was in the 225-50 range for most of the cook. Close enough.
Took 14.5 hours for the butt to get to 195+, and I did not wrap during the cook.
That's a long way from 20 hours, so I think you must be cooking at a significantly lower temp than it appears. It won't hurt it to crank it up a bit.