The light arrow vs heavy arrow stuff is clear enough. How fast the arrows moves depends on how fast the bow string can move. if the arrow weighed next to nothing you would get a speed equal to the string's. As you add weight the string speed slows down, but presumably it is more than a direct relationship since eventualy you reach a point where the string won't move at all. Also you have the situation where weight increases friction as well as directly consuming energy that would otherwise contribute speed, so it isn't going to be just direct.

I was curious about why Jeff thought the parallel limb thing was bad for speed. Is it just the guy running on a flatbed truck thing where you go as fast as you can run, plus the truck's speed? In the case of the Patriot since the limb tips basicaly don't move towards the target, that effect isn't aded to whatever juice you get from the cam doubling and limb motion towards strightening the string which constitute the basic speed engines. Still the bow seems fast.