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Long story short, my oldie MBP 2011' battery started bloating, I've contacted a repair service center and they advised me to remove the battery and use it on AC power, while they get a new battery for a replacement. What I've noticed a few days back, that my MBP starts extremely lagging from time to time. After a restart, it works fine for 12 - 20 hours and then the problem appears again. I've noticed that when the lagging starts, kernel_task begins consuming up to 500% CPU. So I bet that the problem's source is kernel_task, but what causes its extreme rise? Could it be a removed battery?