Your Mac goes all sluggish, you open Activity Monitor, and there at the top of the CPU list is kernel_task, taking 100% or more. Why?
fans
What should you do with a Mac whose fans keeping coming on full blast, with high CPU loads from kernel_task, and sluggish performance?
By default, your Mac’s cooling fans run at full blast. Only the control system slows them down when it is safe to do so. How to diagnose and fix its problems.
My iMac seems to be running a bit hot. How can I check its temperature?
When I started my iMac up, the fan has run up to be really loud. How can I fix this?
The Eclectic Light Company 