some sites are loading very slow on my centos proxy server

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http://www.unix.com – Hello everyone, I've installed squid proxy server in Linux centos machine for users.actually problem is when users browse some websites eg : WordPress.org, gravatar.com it loads very slow compare to others sites .my line is 4mbps and only 6 users for it.I've bypassed squid for myself via iptables .so that websites works fine for me but not for who browse via squid .. please help me with better solution... waiting for reply. centos 5.5 squid 3.0 (HowTos)