I'm going to have to both agree and disagree here
. The definition of octane rating is the resistance to knock. Knock happens when fuel that is in parts of the combustion chamber that the flame front has not reached yet decompose because of the heat, pressure, and light from the fuel that is already burning. As the fuel decomposes it can form species that spontaneously ignite at the temperature the mix has already reached, and so a relatively large volume of fuel mix will simultaneously ignite. This gives a large uncontrolled spike in the pressure in the combustion chamber and can be heard as knock. Preignition or pinging is something else and I'm not going to talk about that now. During normal combustion, as the flame front moves in an expanding circle from the spark plug to the chamber walls, the pressure will rise smoothly and then fall as the mix is consumed and the piston starts going down on the expansion stroke. When knock happens a lot of the fuel mix ignites at the same time when the piston is still very near top dead center, so the pressure can reach over ten times higher than normal, so the forces on the piston, rings, rod, rod bearings, crank, main bearings, chamber, head gasket, and everything else are also ten times normal, which is why knock breaks stuff.
If you take any fuel and simply heat it up rapidly in air there will be some temperature where it will spontaneously ignite. This is called the autoignition temperature and is the property that best correlates with octane rating. As the temperatue rises and the decomposition process begins there are a whole host of chemical reactions that occur that lead to combustion, and anything that slows one of those reactions will slow the overall process and raise the octane rating. Different additives function at different times in this process - oxygenates slow down the low temperature, early reactions, while tetraethyl lead slows down some of the medium-high temperature processes near the end. Neither one is "better", they just do the same job different ways.
As for "how fast high octane fuel burns", I think that people are confusing multiple effects. It is true that higher octane fuel is harder to start burning (the fuel must decompose as part of the combustion process and we just said that high octane means slow decomposition). However, the speed of the flame front as it moves across the chamber is approximately constant for similar gasolines, no matter what the octane rating. So in that sense high octane fuel does not burn "slower" than low octane gasoline. Finally, the flame speed is different for different kinds of gasoline components such as alcohols, aromatics, and ethers such as MTBE, and each of those have their own octane ratings. That means that a gasoline composed purely of branched hydrocarbons will have some octane rating and flame speed, while another gasoline with straight chain hydrocarbons and lots of aromatics and MTBE could have the same octane and much different flame speed. You just can't make any blanket statements about gasoline because there are so many different components in use today (yeah, yeah, that's a blanket statement in itself, sigh
).
I highly recommend locating a copy of the gasoline FAQ put together by Bruce Hamilton if you want lots more info on this stuff. It's available from several places on the web. He also just posted an article to sci.chem last night that has some good stuff in it; Iwill try to post that here tonight if I remember.