Very high kernel_task CPU usage after updating to Yosemite 10.10.4
As far as I am concerned kernel_task was always trying to tell me that the computer is running hot. It seems to be a strange countermeasure which occupies the CPU with a low energy No-Op task which prevents it from doing "hotter" things. So you should be looking for that "other" process eating up the rest of your CPU time.

However I noticed very high CPU usage of Safari ever since I update do Yosemite 10.10.4. The browser visibly (in Activity Monitor) starts several processes now and two of them (Safari Networking and nsurlstoraged) routinely take up a lot of CPU, especially Safari Networking often takes more than 50% of one CPU. It seems to be less constant now, so maybe there is some "trickle down" effect on this one.