How Does a Fast Propagation Delay Impact the Effectiveness of a 51% Attack?

A fast block propagation delay significantly reduces the effectiveness and increases the cost of a 51% attack. Since the attacker's malicious blocks are quickly broadcast, the honest network is rapidly informed, making it harder for the attacker to maintain a secret, longer chain without committing massive, unsustainable hashrate.

Fast propagation limits the attacker's window of opportunity to perform a double-spend.