[shishcat]

I remember having a friend with an extemely sluggish server. Turns out the reason behind this sluggishness was that he set a permanent fail2ban bantime, which added thousands of records to iptabes; any packet, even not SSH, the server had to check the source IP against thousands of records. After purging the rules and switching the bantime the server was usable again.

[pepa65]

This should happen in-kernel and is lightning-fast, so should not be an issue. Maybe a pre-netfilter box..?

[72deluxe]

Big iptables lists are indeed incredibly slow; use ipset for large lists instead.