Diagnose the power supply

There is a device called a power consumption meter, have each computer lab.

If you install the system and there is no improvement, the problem lies in some component not functioning properly.

I recommend you take the machine to a professional computer lab or lab producer.

First check the diagnostic LEDs answered.

If they are good you may have a HD problem -corrupt, full or failing HD's will cause a machine to slow. So will running lots of applications. Unlike WinDUHows when you click the red button on an app (if more than one window can be open at a time), it does not automagically quit. Check the throbbers on the dock, look at Force Quit or the Activity Monitor to see if you have more than your expected apps running).

If this answer is acceptable please remember to return and mark it.