Slowing down the burn means that you have to ignite the mixture sooner than a faster burn. That also means that more pressure is building as the piston is still moving up to TDC (negative work) to allow peak pressure at the most efficient time which is about 12 degrees ATDC. The pressure against the piston on the way up decreases the amount of work the engine can do. That's why fast-burn chambers make more power.

If you could have the mixture burn instantly at 12 degrees after TDC you would have the full push against the piston without the negative push BTDC and make more power. If you didn't have to worry about heat being conducted out of the mixture by the metal surrounding it you could have the instant burn at TDC and get a bit more. Of course this is only theoretical because in real life burning takes time and engines aren't adiabatic - yet. But hopefully that helps explain the need for a fast burn.

Octane is a measure of the resistance to self-ignition or detonation, not how fast or slow it burns.