Macbook Pro lag (need help!), lag on simple tasks

My macbook is a 2012, 2.5 GHz, 4GB.
So recently my macbook has started lagging a lot. After I turn on my laptop, it lags often, it lags doing simple tasks like spotlight, clicking on finder and so on. The spinning wheel keeps appearing always. Not to mention lag on internet. There is no problem about the internet, there is just so much lag on loading websites. The macbook runs fine for few minutes and then there is this moment where it loads for <30 sec and I have to wait, sometimes the spinning wheel appears too. The computer doesnt heat up much, CPU is fine, I reinstalled new OS X and cleared the whole laptop, when I went to the repair service they said everything was fine (the hard drive had no problem they said). Its like an old machine, when you start it up for the first time then everything is choppy, but when i use it for a few hours and the machine gets warmed up its work fine (better). I don't know what to do, i searched a lot and can't find the solution. And I need help because I don't have money to spend on a new laptop.
Might there be a virus or something?