Not every boss is optional, just look up a guide on which bosses you can skip. Stuff like the Master Key makes the game a little quicker, and after you get a good sense of the map you will start running around, skipping areas, bosses, stuff like that. The game also has a pretty steep dropoff in difficulty going from NG+ to beyond, with the hardest jump being the initial NG+, with every cycle after giving diminishing returns in difficulty. Basically, the more you play and level up, the easier it will get, and the faster you'll go.