Upgrade to Yosemite: kextd close to 95% CPU
I had this same issue and I found this message repeatedly on the system logs:
com.apple.kextd[8095]: Kext net.semaja2.kext.insomnia not found for client path request.

Closing InsomniaX quickly solved the issue for me. 
Will do some more research on how to properly fix this in order to have InsomniaX running.

EDIT: After uninstalling InsomniaX and upgrading to the latest build the issue has been resolved.