Computer not running games like it should

Hello,
My computer has - recently - not been running games like it should.
When I play games such as Diablo 3 It won't run at as high an fps as it should and will lag noticeably when moving from area to area taking a much longer time that it should to load.
I also was trying to play the game Runescape and it would not work at all because it would freeze and not work. I don't understand why it isn't working properly and I'm not very

Computer Specs:
Acer Aspire 5755G
Intel core i5 2450M 2.5GHZ
Nvidia geforce Gt 630M with 1GB Dedicated VRAM
4GB DDR3 Memory