I am also experiencing large CPU usage by Avast - it says it is the Behaviour Shield which is constantly using around 50% of CPU, even when I am doing nothing else. I am new to these things so tell me what info I need to provide.
Version is 17.8.2318 (Build 17.8.3705.249)
Running Windows 10. Browser is Chrome
Active Subscription to Avast Internet Security
Paddy