Why does my PC games lag?

Hey guys i have a ok computer here are my specs 550 TI GeForce 1 gb gpu 2.9 dule core processor 8GB ram I meet the recomended requiremts for almost all the games i play but i still get a very low FPS. for example BF3 i used a computer scan from both can i run it requirements lab and the geforce experience website and both say i can run these games maxed/high and yet still i get a low FPS WTF lol. On BF3 i get around 20 fps and arma the same i dont understand. I dont have the minimum requirements i have the maxed and yet still i cant run it so why?