Question with my disk

Hi

My MacBook Pro info: Mid 2012 13-inch (no Retina), 8GB ram(upgraded).

Version : macOS High Serria 10.13.1

Problem: Having problems after formatting my Mac weird. (Game fps goes down)

I had too much programs and junk files so I planned to format my Mac and install Mac OS X back. But it went weird. It was my first time, so I barley installed macOS but suddenly all the games that was fine turnt terrible. It was average 200fps but now its 10~30fps with large spikes, I will upload a screenshot but the formatted disk type was a little different with my normal iMac I have.

Stuff I tried: I tried nearly everything I can do but I don't know what to do. The system is fine but I feel a little buffering.