I recently discovered that off-screen buffers in Java can be really slow. Off-screen buffers are commonly used in Java games so that a frame of animation can be constructed and then displayed on the screen without resulting in flicker. Since the Swing toolkit is double-buffered, most applications don't normally need to create their own off-screen buffers, but I don't like using the UI thread for game logic and rendering in my games (plus my games use AWT), so I have to manage the off-screen buffers myself.
Anyway, many graphics operations in Java are not hardware accelerated yet, and some of the buffer code that I'd been using for a long time has been using some really slow operations; I'd simply never noticed until I started working on a computer with a really old PCI Rage 128 graphics card. With such low bandwidth across the PCI bus, blitting the off-screen buffer images to the screen ended up being really, really slow.
The problem is that I'd created the BufferedImages backing the off-screen buffers as TYPE_INT_ARGB. I think the inclusion of an alpha channel, combined with an assumed lack of hardware acceleration, resulted in a lot of data needing to flow back and forth across the PCI bus unnecessarily. Once I switched the buffers to be TYPE_INT_RGB, everything was back to being snappy and fast.