Dear 100 Hour Board,
Is Superman faster than a speeding Avada Kedavra? Would Superman survive an Avada Kedavra?
My initial reaction (and Anne's, too) was that spells travel fairly slowly - perhaps 60 mph or so. However, I couldn't find the source on the internet that gives the exact speed. That means it's speculation time!
It's certain that the speed of spells is slower than the speed of sound because incantations are heard by their targets before they hit. If magic traveled at the speed of sound (1125 feet/second) and it takes about half a second for a person to speak a three-syllable word (e.g., stupefy, protego), then to block a spell with a three-syllable incantation I would have to be at least 562 feet away from the caster (assuming I didn't start casting shield charm before the caster was done saying the incantation). There's no way anyone could hear an incantation at that distance unless they were yelling.
It seems reasonable to assume that duellers frequently stand about 10 feet apart. It would take the sound of the finished incantation about .009 seconds to travel that distance. We need the spell to take at least 0.5 seconds to travel. So the spell needs to take a maximum of 20 feet/second to travel that far. Or about 13 miles per hour. That's too slow.
Okay, that's not good. Let's assume duellers who have any sense of caution stand about 20 feet apart. That would double the speed it would take to cross that distance in .5 seconds to about 40 feet per/second or 26 mph. Also slow.
Okay, let's pretend they stand 30 feet apart. That brings the required speed up to 60 feet per second or 39 mph. Also still slower than Usain Bolt.
We made some big, not totally realistic assumptions conducive to spell slowness (i.e., that people being don't react before the incantation is over). I don't really see the number getting much higher than 50 or 60 mph (approximately 80 or 100 km/h). There. Final answer.
If anyone would like to submit a correction because they have a link to the quote that Anne and I were thinking about or they noticed my math is terrible, please do so.