Strange kernel logging

I was playing with nmap and tryed to do portscan on my 2.4.18 kernel with iptables.
I use port -sS -p 1 to search all ports. And I have iptables to do logging on all
rejected messags. The scan mod was "norlmal" and that sends messags in bursts.
And when this burts was recived the machine was very slo and had keyboard problems.
It was very low latency for the keyevents. It was funny to see what happend when
key relase event was delayed. A lot of same chars... This is on a dual PII and
I guess that it is some race condition with in the logging in iptables. Is this
a known problem? Any one that have solution?