Seperate image file vs Sprite sheet

I’m about to add some images to my game and i was just wondering if having separate image files was faster than having a sprite sheet. I know the obvious reason for having a sprite sheet would be less memory but for right now, my game isn’t really memory heavy and the sprites I’m going to use aren’t very heavy either. I know that mincraft just recently changed to all separate images for it’s sprites and I didn’t see any significant difference in performance after it happened.
Thanks in advance.

btw: I’m using Java2D

Sprite sheets are more efficient because you only have to load them once. Then, you can just cut them up. Individual images have to be loaded individually, obviously. More here: http://gamedev.stackexchange.com/questions/7069/2d-graphics-why-use-spritesheets