I somewhat doubt Legilimency seriously helps in an intense duel between two closely-powered wizards that has any Occlumency training. After all, who the hell has the time to batter down someone's mental walls when the other side is firing AKs? The most you can get is a slight distraction, but you get distracted yourself too.

 

Lulz. Fandom Occlumency sucks balls.

I always saw the duel between Dumbledore and Voldemort as being accompanied by an invisible, silent duel of their minds which we never saw. That said, in this particular duel I don't think it made much of a difference, because they were pretty much evenly matched.

 

Just because it's not Unforgivable doesn't mean it's not still illegal.

 

Not everything that is luminescent moves at the speed of light.
Spells are not made of light, they are made of energy, which gives off light. It's probably more like fire or plasma. Flames give off light, but they aren't made of light. (lord help me if a debate starts on waves vs particles and such...)

If spells moved anywhere near the speed of light, then magical duels would be based entirely, not just a little but entirely, on instinct and predicting what your opponent will do a second before they do it. If that were the case, I think it would have been at least implied in the books. All that talk of defense in the DA and no one mentions the vital necessity of predicting your opponent's next move? The only whiff of this we get is when Snape shits all over Harry's parade at the end of HBP by using his Legilimency to know what Harry's going to cast.

I'm no scientist, so perhaps I'm wrong, but I don't think there is an abundance of data on how fast a ball of plasma or pure energy does/can move when fired from an object. The shit doesn't last long in the open air and I imagine it would dissipate even more rapidly if it were moving quickly. This (and the lack of a portable source of fuck-tons of energy) is why we don't have light sabers yet. Which is a shame, because lawn darts and micro-uzis really need a twenty-first century replacement as 'most stupidly deadly child's toy'.

Lacking the previously mentioned data, we have to fall back on 'because it's magic' (and rightly so; applying science to magic almost always signals imminent failure). Being that it's magic, we could conceivably say that different spells move at different speeds.

That being said, I must reiterate that if all or even most spells moved at the speed of light, Voldemort would have either conquered the world or fallen to an auror's wand ages ago. There would be virtually no defense against something like an insta-kill spell moving at (or even in the same zip code as) the speed of light. Yes, it takes time to say the incantation (unless you do it non-verbally) but that's a very slim margin of error.

Look at the number of spells cast at Harry throughout the books, then compare them to the number that actually hit him. If spells moved at the speed of light, he'd have been worm food a couple of books into the series.

 

And even using fandom "jedi-minding-reading-legimency", all you would need to defeat someone is to distract him for a moment with the mental arts, thus overcoming him with some well placed strategy.

 

From my prestigious background in sci-fi, I can say that although people often wank over 'plasma guns,' they do so because they don't realize that a plasma gun would be the same thing as a hot air gun. This, of course, does not sound as cool.

 

When I think of spell traveling speed, I think of intention as being a weighting factor. Say you wanted to shield from a dark spell; you would put up a supposedly light shield. The spell itself would be light and travel fast (and God was that a terrible pun). So I assume that while Avada Kedavra and Crucio, while not being slow, would be slower than light spells. If only because otherwise the dark would've won eons ago.

The reason people like Dumbledore don't shoot stunners out of their eyeballs is because it takes more energy than shooting it out of a pointed magic stick would. Nonverbally casting Avada Kedavra would take a lot of concentration; the fox example is easily explained in that it was offhanded and it wasn't in a split-second scenario.

Harry Potter magic is a moot point anyway; physics don't apply the same way. If they did there would be more original spells that didn't suck wrinkled nutsack.

 

It's not as if Avada Kedavra requires more power (as in magic points/mana/energy/whatever), but more concentration. For instance, take the Patronus Charm for example. It's a very difficult spell to cast - not because it will exhaust you, but it requires to have a very specific, very strong emotion. But, you will not be magically exhausted after you cast it - there is no magical exhaustion in the Potterverse, or mana, or magic points, or energy, or anything of the sort. Rowling's system is a bit more clever, and IMO, better - spells can be more or less difficult depending on one's concentration and intent. Likewise, I suspect the AK, as hinted by Crouch and others, requires very strong concentration and likely intent to kill. Not necessarily hate (as that is never mentioned), but a very strong desire to kill. That can easily bring hate along with it, but hate isn't absolutely required. For that matter, I doubt you need burning hate to cast the Cruciatus - just a strong desire to cause pain, which may or may not accompany strong hate viz. Harry's casting of the Crucio on Carrow in DH. I doubt Harry really hated Carrow all that much, but desired to cause some intense pain.

As for the speed, who knows. I prefer the idea of it traveling as fast as a bullet or the speed of light, but in OotP, Fawkes manages to get in the way of Voldemort's AK after it's fired and Dumbledore also animates a statue to block one after it's fired. But, that just may have been Fawkes and statue moving just before Voldemort finished saying the spell. I imagine it would be very fast though, either way.

 

The speed of light may be fast, but it not the fasted of things. No matter how fast light travels, you will always find that dark was there first. The light may drive the darkness out before it, but the darkness comes before the light.