Why does my mid-2010 Macbook Pro suffer from huge FPS drops whenever I try to play games?

Question

Macbook info: 2.66 GHz Intel Core i7, 4GB 1067 MHz DDR3, nVIDIA GeForce GT 330M 512MB
When I play games on my Macbook Pro, such as League of Legends, Diablo III, World of Warcraft, etc., they operate well with latency of ~80ms usually and fps starting off between 30-60, but after only a couple seconds or minutes my fps drops to 5-15 and the games become unplayable and I must force quit the program. Restarting the computer helps temporarily but then it relapses.
I cleaned out all my downloads, applications, and other deletable files that I could find. I installed and updated Mavericks OS X and the issue persists. Before I installed Mavericks I noticed in Activity Monitor that almost all of my memory was in use while playing a game. After Mavericks the memory is almost all permanently in use, but from what I've read this is what is supposed to happen and instead of looking at memory in use you look at memory pressure, which looks normal.
I was going to upgrade from 4GB RAM (current) to 8GB RAM and see if that helps but idk if that has anything to do with my fps issue or not. Any thoughts?

Answer 1

Alright well I'm definitely not running virus scans or doing system backups and then complaining about slow performance.... The only other application running besides whatever game i'm playing is SMC fancontrol to make sure my computer isn't overheating. 
In terms of CPU%, games like WoW and LoL take ~50-60% of CPU while active, but activity monitor still reports ~80% idle. For disk activity, i'm not sure whether it uses optical or not, I assume i just uses the hard drive, which is brand new cus it broke down about a month ago and I had to replace it (the fans are brand new as well, so I don't think it could be build up of dust or anything inside because it was opened and cleaned so recently). 
I can set all graphics setting to lowest possible and improve my fps to 20, which is still awful, and enabling vertical sync just sends it back down to 15 fps. I don't see why I should have to use lowest graphics settings anyway. It can't be out of date drivers since i have 10.9.1 running. Could the graphics card be the issue? That would suck since I can replace and upgrade RAM but not the graphics card.
I have repaired disk and disk permissions multiple times, virus scans, the whole gambit and nothing seems to help.