Mac Running Slow, Won't Shut Down

Hi, my Macbook Air won't shut down, it's just a black screen with a loading icon on the bottom. It also runs very slow, but it says my internet is 4/4 bars. The internet loads very slowly, and I have to constantly refresh it. Occasionally when I play multiplayer games like Minecraft, I normally get about 60 fps max, and now I only get 24, and sometimes only 12. I have been playing other games like ROBLOX recently, that's when the problem started.

System:

Macbook Air (11-inch, Mid 2012)

Processor - 1.7 GHz Intel Core i5

Memory - 4 GB 1600 MHz DDR3

Graphics - Intel HD Graphics 4000 1024 MB

I also have 239.2 GB on my startup disk, and there is 68.9 GB free. I modified the startup disk a bit.