Memory Limit


I have a CentOS server, which has i3 CPU and 8GB Ram. 27 Virtualservers are placed in my server. Just 3 of them have medium traffic (300K pages/month). Unfortunately the web page loading times are very high now. I don’t know what happened, but I see allways that the Realmemory usage is near and less then 20%, which seems it is caused by a limitation.
I have reloaded Apache2 and restored all web sites, but nothing changed.
I have installed Squid, also nothing changed.
One of the web sites I mentioned is
Any help is valuable for me Thanks in advance