If you have a time machine backup of your system (running El Capitan) you just restart the computer in recovery mode and then choose the option "restore from time machine backup". But make sure you copy all the new or altered files to an external harddrive because those will be lost when going back to a previous version of your system.
Process 'secd' is the "security daemon" running iCloud Keychain. It should always run but not at 75%. Try logging in and out of your iCloud Keychain, force kill the process and reboot your Mac. Stay on Sierra the OS itself is not the problem.
Thank you !
Finally a solution ...
System Preferences-Icloud-Keychain ( set it up and problem solved)
secd process was running all the time until i set up the Keychain.