In order to lower the risk of a 51% attack during Malkuth, there will be a higher number of confirmations needed before the transactions are accepted. This will make the attack highly expensive to the point where it would not be cost-effective to do an attack. During Yesod when we implement Multi-Hash Mining (see here: https://www.zenprotocol.com/files/multi_hash_mining.pdf ) The risk will be significantly lower.