I experienced the same problem after updating Avast to the new version. CPU was above 50%, 8Go of Ram were at 97%, the SSD was at 100%, and even the GPU was at 20-30%, and all those things without anything open. Obviously, i saw the problem after my PC froze a first time and i had to shut it down manually. (My PC is Windows 10 by the way.)
Thanks to Zemo post above, it conviced me to do the same, by following the process to uninstalling Avast and reinstalling it.
After reinstalling Avast, i restarted my computer (I'm cautious^^), and thus far, it work normally, with CPU at 1-5% and Ram at 30% when nothing is open, which is my normal as far as i know of it.
For today, i shall consider the problem fixed thanks to clean unistall/reinstall of Avast.