this would make for an experience that is 75% clearing a dungeon (the fun part) and 25% killing a boss, which can be fun if it doesnt drag on for 15mins. So you reach your pit limit for your gear/build when you are unable to clear fast enough as well as kill the boss fast enough.
Right now, you can clear a dungeon with 10mins remaining (so you spent 5mins doing the fun stuff) and now you are on a boss fight that will take so long you wont finish it before the 10mins are up…Is that fun? not remotely.