open up the case and look inside, see if any wires/cables are rubbing the fan blades and slowing it down.
Also, remove any huge dust blobs with your bare fingers (after grounding), by slowing pulling dust off from inner edge to the tip of each fan blade. Do this VERY GENTLY, don't exert large forces on the fan.
If this does work, simply unplug and replug the fans 4pin connector.
Best thing however, if your comfortable with it is to pull out the entire fan and inspect it properly and clean it