Why is my mac very slow at operating and fan very loud!

My MacBook Pro 13" unibody late 2011 core i5

Runs very slow at booting and even when the laptop is fully on everything is basically slow,the mouse drags alot and sometimes I get the spinning color wheel but all the time the mouse moves very slowly even when track pad speed is set to max.speed and the harddrive has nothing on it to slow it down because the harddrive is empty.and the fan makes on !&&* of a noise that even when I installed smcfancontrol the fan still makes noise and the laptop isn't over heating.

The moment I power on the laptop the fan starts making noise.ive tried lots of stuffs like performing fresh osx installation even when I perform installation it runs slowly I've changed the harddrive severally and I've double checked the ram.and the ram is 8gb all seems not to work.but when I use boot camp on windows it runs normally and fan control slows down the speed that I was forced to be using my mbp like other windows laptop.the moment I install mac the slowness and fan noise kicks off again please what do I do