An upcoming device actually makes your GBA run faster (or slower).
Ahead Games is developing a device they call the GBAccelerator, which basically overclocks your GBA and allows it to run much faster than normal. It can also slow down the system's performance. So why would you want to alter your GBA's processor speed? Well, since games literally run faster or slower, you can add extra challenge or cheat a bit on games that are primarily time based, like puzzle and racing games. The function can also be useful for homebrew game developers who want to squeeze more performance out of the hardware. There are four modes in the device: slow, normal, fast, and very fast. Each mode is indicated by a blinking pattern in the GBA's battery light.
Ahead Games has posted some more information in their forums, linked above. There are also a number of videos of the device being used with homebrew games and some commercial software. The company says development on the device is nearing completion, so it should be on sale soon. Please remember that any hardware modification voids the warranty on your system.