I have the same problem. The firewall worked well in january with full performance (almost 1 gigabit), but after an update (I don't know which, because I didn't check the performance until march) it halved the performance at 100% cpu usage. Suricata processes eat up the cpu (/usr/local/bin/suricata -D --netmap --pidfile /var/run/suricata.pid -c /usr/local/etc/suricata/suricata.yaml{W#03-igc1^}). If I remove all the rules (and apply, restart), the cpu load remains the same when I run speedtest.
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