Macbook Pro Mid2012 Lags 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.
If you run Yosemite or El Capitan, enable the native OS X trim support with the Terminal command : sudo trimforce enable, followed by a restart.
For older OS X versions try the TRIM enabler from Cindori Software.