I am attempting to lower the process priority of the AvastSvc.exe but am unable to because of an Access Denied error. The Disk IO of Avast Pro is killing my machine especially at start up and takes quite a long time to calm down. Why is there an Access denied error when attempting to lower the priority of the process when I am admin on the box? Are there settings in avast to manage performance?
You can’t (and shouldn’t really :)) change the avast! process priority if avast! auto protection module is on.
By the way, reducing the priority will make the scannings take longer, and the whole computer performance will decrease. No gain on it.
Go to settings > troubleshooting
Untick the self defence mode
Apply any changes
Reset self defence
But you know he shouldn’t do it, don’t you? 
Yep… But once the slow scan becomes apparent he can change it back