That is the whole point of having an on-access resident scanner to scan files before they are run to prevent malware from running.

It isn’t scanning files when the system is basically idle, avast only scan activity (newly created, modified, run, etc… Even then it is only certain files that will be scanned, executable files and those which can be infected and would represent an immediate risk if run.

Have you made any other changes to the file shield scanner settings ?
Do you have any other security application installed ?

Avast isn’t designed to be an on-demand scanner.