I'm fine with speed runs existing, but I think they should be for achievements only. 5th chests should be difficult, but not based on speed runs. Maybe make certain conditions that will allow for bosses to have a hard mode. I like the idea of having objectives that are optional within the dungeon that would activate a sort of "normal" mode on the boss.

If you skip that objective, you'd face the hard mode of the boss, and would be able to get the 5th chest by killing him.

But in general, I don't like feeling rushed. It would be nice to just take my time through an instance for once. Kill enemies, have fun, not always feel like I need to put the lead out so we can make our speed run.