Java2D is already by nature quite slow. That's why there are alternatives like OpenGL, via LWJGL or JOGL. However, there are many tweaks you can do to make your app run at the fastest possible.
One thing I can think of is using compatible BufferedImages. This is done using some long static methods:
GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment();
GraphicsDevice gd = ge.getDefaultScreenDevice();
GraphicsConfiguration gc = gd.getDefaultConfiguration();
BufferedImage compatibleImage = gc.createCompatibleImage(width,height,transparency);