I had recently a problem with the battery of my MacBook Pro (MacBookPro8,2), so I decided to remove it (full story here). From that point on, every now and then, the kernel_task started to use 200-300% of the CPU (according to the CPU monitor), and the whole system became really slow. The only solution at that point was to restart the machine, but the problem appeared again after 1-20 minutes. I put the battery back and it fixed the problem for good.
My guess is that the battery is important for the air flow within the machine. If it’s not present, some parts are getting too hot and the system slows itself down artificially. That’s only a guess, but it might help you if you have to face the same symptoms.