Im sorry for asking so late but I hope it can help someone. I had this issue in my Mid 2012 1,8 GHz Intel Core i5 Air and in my case, it was a faulty sensor in the board. Constant fan, it worked VERY slowly. Activity Monitor was telling me that a process called "kernel_task"was using 87-93% … Apparetly it´s a common issue in certain Airs and MBPs. I got it fixed by an independent repair , $120 aprox, and it works like new.