Squid Guard poor performance!!!

[mafiosa]

Hi !!! I have installed squid proxy server(transperant mode) and squidguard mainly to block porn thats all.I do not need caching!!!When I configured squid with filesystem "null" in local cache the system hangs when I try to download something big around 300mb.Moreover I have allowed 900mb of ram to squid for transit.I also have clamav running.After sometime i face poor speed in downloads around 1/3 rd speed is what I get.Did no tweaks,please help me.Should I keep the squid file system in "null" format or change to ufs or diskd?
I am using a 5400rpm 1tb harddisk.With clam-av ram usage is around 38-39%

[mafiosa]

Someone please help!!!