Detailed examination showed, that process called kernel_task is generating very high load.
After searching through multiple discussion forums and apple support site I have found, that the high load of kernel_task could be related to Bluetooth.
I'm using wireless keyboard and magic mice connected using Bluetooth, so I can not switch it off permanently.
After messing around I've found, that simply switching off Bluetooth for 1 minute and starting it again solved the problem and kernel_task dropped to 1-2% CPU load again.
I think the problem could be related to fact, that my keyboard battery was very low.