The kernel is using excessive processor cycles. Below is a partial list of causes for this condition.
Throttling
When it gets high temperature readings from the hardware, or a low-voltage reading from the battery, the kernel may try to compensate by interrupting the processor(s) to slow them down and reduce power consumption. This condition can be due to a buildup of dust on the logic board, to high ambient temperature, to a worn-out or faulty battery, or to the malfunction of a cooling fan or sensor. Note that if the problem is caused by a sensor, there may be no actual overheating or undervoltage.
If the computer is portable, test with and without the AC adapter connected. If kernel_task hogs the processor only on battery power, the fault is in the battery or the logic board.
The Apple Hardware Test, though not very reliable, is sometimes able to detect a bad fan or sensor.
Installed software
User-installed software that includes a device driver or other kernel code may thrash the kernel. Some system-monitoring applications, such as "iStat," can also contribute to the problem. You can test for this possibility by completely disabling or removing the software according to the developer's instructions, or booting in safe mode. Note, however, that disabling a system modification without removing it or booting in safe mode may not be as easy as you think it is.
Corrupt NVRAM or SMC data
Sometimes the problem is cleared up by resetting the NVRAM or the SMC.
Peripherals
According to at least one report, connecting an external display to a MacBook Pro via a Thunderbolt-to-DVI adapter may cause this issue. I can't confirm.
Encryption
Transferring large amounts of data to or from an encrypted disk image or FileVault volume may put an extra load on the kernel. If both the source and the destination are encrypted, the load may be doubled. If you transfer data from an encrypted disk image on an encrypted partition to another such image on another encrypted partition, the load may be quadrupled.
This issue probably doesn't affect late-model Macs with an Intel i-series, recent Xeon, or later processor. Those processors support hardware-accelerated encryption. You can determine what kind of processor you have by selecting About This Mac from the Apple menu in the menu bar.
