I have a macbook pro mid 2012 and recently I upgraded it by changing the RAM to 16GB and HDD to 500GB SSD. It was fine for a while but now it lags a lot whenever I play any games or even when I am normally using it. I did Memory and Hard drive test and both of them were fine. I even replaced my RAM because I read it somewhere that I should replace my RAM when these things happen, but still the problem occurs.
While upgrading my Hard drive I cloned my old hard drive and then transferred that to the SSD, I don't know if that's what causing the problem.
Best Answer
You also can try re-instal the osx using the: cmd+r at the start, see this video: https://youtu.be/vxSbpZAIGm8 , Isn't mine, this can help you