CPU usage is too high with Yosemite and an external monitor
This is a design feature documented by Apple. See if cooling your Mac and the processor can then speed up again and do the work you want. See this post on ideas for cooling while in clamshell mode.


Any reason not to run my Macbook Pro upside-down in clamshell mode?


Some people have luck removing Intel kernel extensions, but if you disable safety and overheat your system, that might be out of warranty if the cpu fails. 


rMBP kernel_task spikes when connecting more than one external monitor


I would watch things closely if you do that and try other cooling methods first, like buy a stand designed to cool your Mac.


http://www.svalt.com


If that doesn't work, try resetting the System Management Controller one time to ensure it's not a measurement error.

If you only need occasional cooling or to convince yourself, go ahead and open the clamshell and make sure there is good ventilation and you are not blocking the air inlet or outlet by the display hinges. The thermal management software reacts quite rapidly to changes in internal temperature.  could even use Magnets to force the display to sleep while it's actually open and allowing the entire keyboard surface to be exposed and cooling the Mac that thinks it's still working in closed clamshell mode.


https://support.apple.com/en-us/HT203184



  Activity Monitor may show that a process named kernel_task is using a large percentage of your CPU, and during this time you may notice a lot of fan activity. This process helps manage temperature by making the CPU less available to processes that are using the CPU intensely. In other words, kernel_task responds to conditions that cause your CPU to become too hot. When the temperature decreases, kernel_task automatically reduces its activity. 


So, also quit the other tasks and apps you don't need if you can't make the physical arrangement of the Mac more cool or more conducive to radiating the heat away if convection isn't working for you. The GPU will cause lots of heat if it's pushing many pixels. Decreasing resolution might help in an edge case, but you'll want to attack cooling in general as a first step.

You could also get some advanced tools to peek into the CPU rate to confirm it's reacting to heat:


kernel_task hundreds of % CPU yet cpu frequency is being lowered