The backing group size on Kusama (and Polkadot) has been set to 5 (configuration->activeConfig->maxValidatorsPerCore). This proposal is aiming at reducing the group size to 3.

Currently, when a parachain produces a block, it requires 2 out of 5 backing validators to verify and sign it in order to back it and move forward. With 2-of-3, we'd guarantee that backing quorum is a group supermajority, so there're not two competing 2/5 backed candidates. And one extra validator is allowed to be slow. Smaller backing groups should also contribute to distributing the load of approval-checking more evenly.

There's a small risk, however, that with slow/unresponsive validators, parachain slots might be skipped more often with 2/3 instead of 2/5. This should not be an issue in practice since validator groups rotate every minute.