Go into Avast GUI on your desktop. Select Real Time Shields - Web Shield.

Next select Expert Settings. Then checkmark the setting: “Scan traffic from well know processes only.” Then reboot.

With this above setting, avastsrv.exe should only show CPU and disk I/O activity while your browser is active. If your browser is closed or inactive, you should observe little CPU activity from avastavc.exe.

Finally, at peak browser activity times such as when your browser loads or when accessing your home page, CPU activity could be high. However, that should drop off radically once your home page is loaded, etc.