Computer really lags while playing games?

My computer really lags while playing games, I have a "gaming computer" or a laptop that is made for games
My Specs are:
8GB RAM
Intel Core i5 2430M CPU 2.40GHZ
nVidia GeForce GTA6540M CUDA 2GB GPU
1TB HDD
-
I did a RAM and HDD test on it, no errors.
I have also updated the latest drivers for my graphic card
It lags at these simple games:
League of legends
Garry's Mod
Skyrim
Minecraft
Crysis 2
and more
not always does it lag but it has horrible lag spikes, including single player.
What should I do?

Minecraft shaders mod problems?

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How to Make Minecraft Shaders Not Lag At all?

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Added (1). I have not installed new mods; I got a new computer in the past two years, so nothing new, .minecraft is there, but I don't find any reason to uninstall it if I just installed it.

How to have different minecraft modloaders on same pc?

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I know i see multiplayer on the main screen, and I already have servers that i downloaded online, like mod servers with multiple people and custom games.

But how do you play Multi-player Survival and Creative mode with friends?

I played mine craft on xbox360 at a friends house and we all had split screens and built stuff together in survival. How do you do that on PC/mac?