i've never had a sim take that long unless something was wrong.  maybe it's poly count but make sure

• you only have the necessary body parts collidiing.  in this case, chest, torso, hip, thighs, legs.
• that your dress doesn't touch her body in the initial state.
• that you're simulating only what you need to.  iirc, if all you need is draping, RDNA has a tutorial for just that.