Because that are 3D accelerated plugins, and use 3D APIs to draw the graphics. In fact, that's a question you should address to ATI, as they are the ones that design the card with 3D in mind. Nowadays, graphic cards are focused mainly to move zillions of polygons and apply tons of fancy lightning effects on them, so it's no wonder 3D is much faster than 2D. You might want to try PEOPS spu plugin, as it's a directdraw plugin, and is faster when using 2D.