I've done speed runs mostly because killing trash is nothing more than a waste of time, if perhaps they dropped random items/gil or equipment (which isn't vendor trash) and vendor trash people may be more inclined to actually kill them. With there being a timer set on dungeons speed runs actually allowed for learning of bosses and boss mechanics, removing the ability to do speed runs for some groups actually removes attempts on bosses and leads to not completing the dungeon.

I've been in both 50 dungeons where when I was learning the dungeon and bosses I killed all the trash, doing so lead me to never complete the dungeon as I spent more time on trash than I did bosses. When I started doing only bosses and no trash I completed the dungeon every time, simple solution is to make the trash worth while to kill and also a reduction in difficulty to allow new visitors the chance to actually learn bosses before the timer ends.