You didn't say what version of Outpost firewall you are using, if on the Pro version and there have been two updates recently, one of which (Outpost Firewall Pro ver. 7.0 and 7.0.1 ) included a number of different security measures/functions.
However, I haven't noticed any performance impact with them. They have just released Outpost Firewall Pro ver. 7.0.2, which I have downloaded 28.3MB (a pain on dial-up), but have yet to install after I do my weekly drive image backup.