(You're from India right, a place of such a poetic heritage! What happened to your name? ;)
I think the combination intel core i3, DDR3 4 GB, 500GB SATA HDD and Ubuntu (Server Edition?) is capable of more. But from the browser requests to the Moodle pages delivered and displayed it is a long chain of events. You need to find out, isolate, the weakest link and eliminate it. If still slow the next weakest, and so on. To do that you need some understanding of the client-side, the network and the server side. Nobody can tell you the full story here. But all the initial pointers are reachable through https://docs.moodle.org/27/en/Performance.
For more information on the general approach, click on the "Add a new discussion" button in this forum.