Yeah, or dust - my C2D Macbook Pro used to peg all its cores with kernel_task at the slightest hint of work, to the point where the UI thread started hitching because it was getting preempted, and the mouse would skip around the screen. Took me a bit of research to figure out that they use kernel_task for thermal throttling (great naming, guys). Opening it up, there were mats of dust that looked like felt pads between the fans and the heatsinks. Took out the felt pads, suddenly everything was smooth as butter.