You say, "Slower ignition". Hence in other terms, more resistant to ignition. Not slower burning. If it were slower burning, we'd have some problems. Like lack of ability to get to high rpms. If your engine is spinning at 6k rpms burning 87 octane and you introduce a slower burning fuel, you lose rpms. The fuel would not be able to keep up. Think of an Indy car, spinning 18k rpms. If the higher octane fuel they run was slower burning that a normal 87, they would never reach the insane rpms they reach. Octane is just it's resistance to burning, not it's rate of burn. Lead in fuel does not slow the burn rate, it smooths the flame front. Allowing for a more even burn vs and uncontrolled burn, which leads to detonation. The reason why we have high octane fuel, to combat preignition and detonation. Allowing engines to run hotter with a lot higher compression. Which is able to extract more of the heat energy and change it into mechanical motion. Lead absorbs some of the heat, allowing for the burn to be controlled instead of going off like a bomb. Lead also makes an excellent lubricant. Now a days we use other chemicals to fulfill the role of lead. Chemicals that will absorb more heat before they crack and release their energy. Resulting in a smoother flame front and ability to handle the high compression ( higher thermal load on the fuel charge) before igniting. If we want to talk about burn rates, we can also talk about reloading firearm rounds. In those you pick a fuels burn rate, or wind up messing yourself up or destroying your firearm.
Remember at 6k rpms, the engine is firing 6000 times a minute( 60 times a second). If the fuel burnt slower in a high octane engine, high rpms would be unattainable with current designs. We would need huge stroke lengths to take up the slack of a slower burn rate.
