Say for example a two man, thief/blaster group with no tank to absorb damage, if the mob splorks, it's essentially a free kill. If the mob dies on blasting before the first round, it's extremely good value. But if the mob lives to deliver hits, a run can be slowed down significantly.
Aside from all the suggestions of broken game mechanics. I'm tending to believe that the growing xp is due more to increasing refinement of playstyles than anything else. When I was levelling my first char, It was normal enough to top (or come close to the top) the weekly xp list with a 2b week, and those weeks weren't easy. Arguably, few game mechanics have been drastically changed since then, and people are beginning to run xp near the tens of billions.
Perhaps Sloth needs a difficulty boost at a fundamental level?
And no, I have no idea how that could be done, but it's a theory
