Consistent Console error message: invalid signature for com.cisco.kext.acsock
I had the same issue. kextd was consistently using around 14% CPU and tailing of /var/log/system.log showed that it was trying and failing to load com.cisco.kext.acsock over and over again.

I uninstalled the Cisco AnyConnect VPN client using its installer caused the problem to end immediately.

I suspect I had an old version of AnyConnect installed that was from the pre-El Capitan days, and hence it didn't have a valid the signature. Luckily I don't need the AnyConnect client (I had installed it a while back briefly for some troubleshooting) since I use the built-in client.