Starting to fall outside of my domain so I encourage correction however one of the reasons, from what I understand, is that iptables resolves rules sequentially (top to bottom within the chain) versus IPVS which is a hash table lookup—the more rules, the slower iptables gets. The way that the loadbalancing works with the iptables implementation is rules with decreasing probability. That means that, worse case, the kernel with match and fall through one rule per replica. In other words, with 20 pods that match a service selector, 20 rules will be processed before the packet destination is rewritten to the right pod IP and port. Not a big impact when you’re dealing with a few rules but a tangible impact when you’re dealing with the amount of rules created by a good-sized kubernetes cluster.