cpu goes +80% when speedlimit is on

[Hetfield]

No such thing here, do you have any software firewall running? Alot of these causes much problems with these things.

EDIT: Oops, you're indeed running Zonealarm Pro. In my experience this software is really a pain in the *ss. Perhaps you can try to really uninstall it (just disabling won't work) and try if it still happens.