While using squid proxy i am able to receive only 3-4 mbps out of 10mbps

I am able to receive only 3-4 mbps out of 10 mbps internet speed on my router interface. Right now we are using squid proxy and we have 400+ users. I used trial version of check point firewall and every thing was fine. Now it has expired and we are back to proxy. It is slow again. Can any one help with where can my configuration be wrong?. Have to use proxy until we can purchase a new firewall