The primary difference between Polkadot & Kusama's speed is that Kusama has modified governance mechanisms that enable faster upgrades. Kusama runs 4x faster than Polkadot. In terms of block speed, Kusama's blockchain is 4 times faster than Polkadot. It takes less time to go between governance events. It allows Kusama, unlike Polkadot, to evolve and adapt faster.