By default squid will only allow 5 connections to squidguard if u have more than 5 users at the same time u are doom.
Now that u are in your shell check cache.log and see if u have something like this:
2013/10/25 09:44:24| WARNING: All redirector processes are busy.
2013/10/25 09:44:24| Consider increasing the number of redirector processes in your config file.
If this is your case, just remember a formula:
of Squidguard threats x memory size each one=RAM memory for squidguard.
Because squidguard depends on RAM that is why is fast.
Hope this is your case and hope this helps!!!