Recent performance issues with gaming?

I used to get a high FPS in singleplayer modes in all of my games, now I only get a high FPS in multiplayer modes, and a horrible performance in singleplayer modes. This has been happening with all of my games, and I do not know why. It never used to be this way.
I will use Minecraft as an example, I used to get an FPS higher than my monitor would even support, now I only get 30-55. Before I used to play the game with shaders, now I can only do that in multiplayer.

Do you overclock? Is your computer getting too hot? Virus? Outdated software? Old Hardware?

Likely one of those, may be something else though.