I noticed today that my Mid-2009 work-provied MacBook Air, which I recently freshly installed with Lion, was generating a ton of heat, and operating really slowly. I noticed that the
kernel_task process was using between 130% and 150% CPU, and the CPU meter was completely full. Meanwhile, no other processes were using a significant amount of RAM and CPU (in fact, I have barely installed anything on this machine yet). I tried quitting all of my applications, unplugging all peripherals, and disabling wifi with no change. I rebooted with no change. Even researching the problem online was highly frustrating because things were going so slowly.
After reading a variety of message board postings I came to the conclusion that the
kernel_task high CPU usage was a symptom, not a cause. The machine was overheating due to a combination of very high temperatures (high-90s F) and being seated on a surface that did not allow it to dissipate heat effectively. The
kernel_task was running, apparently, to keep other processes from using the CPU.
Strange as this seemed, there was an easy test — I plopped the laptop, now running nothing else, in front of an air conditioner and hit it full blast. Within a few minutes the
kernel_task process dropped to 70-80% CPU usage and things became responsive again. But even though the laptop was cool to the touch, the process never went down from there.
My next thought was that perhaps the new FileVault full-disk encryption was playing a role, since it was one of the only things running once I had disabled all third-party processes and quit all apps. So I set the hard drive to decrypting, which took only 20-30 minutes on SSD. Sure enough, as soon as it completed, the
kernel_task processor usage dropped to almost nothing.
I’m not going to repeat the experiment to verify the results, so use this as one data point only. But if you are experiencing similar behavior, first cool the machine and move it to a hard, flat surface. That should alleviate most of the symptoms.