In XDPoS, only the Masternodes or the block producers can create blocks. Since transactions execute with just a few witnesses, speed increases, costs are cut, and scaling is improved. The witnesses are closely monitored. If they engage in bad behavior, the Masternodes can disqualify them for the following round. For example, if a witness attempts to censor a transaction, or if they double-spend, the remaining Masternodes can "fire" the culprits, as it were, by slashing them for the following round.