It takes one shot every two seconds, which is what I expected, then after the 11th or 12th shot it clearly slows down to around four seconds per shot. Is that normal? What can I do to keep the interval consistent?
Without having looked at the script, I'd suggest that entering an impossible interval isn't a good idea. I wouldn't be at all surprised if that breaks something. Figure out what your camera can actually maintain and use something slightly more than that.
You can test this by using canons own continuous mode and holding the shutter down.
According to
http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&tabact=ModelTechSpecsTabAct&fcategoryid=221&modelid=18155 your camera can do 0.8 FPS continuous. Almost all canon P&S can do the continuous rate indefinitely, assuming the card can keep up. Unlike many P&S, they generally don't do a few shots quick and then slow down when some buffer fills up. Your symptom sounds a bit like this, so it's possible the a480 is an exception, or something else is causing similar symptoms. Testing the built in continuous mode should help you figure this out.
edit:
The manual for your camera does say this about continuous shooting:
As the number of exposures increases, the shooting intervals may take longer.
