For the mid-dungeon chests, it can be tough to roll immediately if you want to compare the item to what you already have, but you're in the midst of combat at the time. Few parties will wait for loot distribution before moving on into the next fight. I think that's why it gives you a while, and on those mid-dungeon chests it does kind of need to.

It's mostly on the final boss's chest where the wait for it gets to be an issue. So maybe the timer for just that one could be a bit shorter.