Avast On-Access Scanner is the resident, on-access, part of the antivirus, the most important one. It is represented by the ‘a’ blue icon.
VRDB should be generated, by default, each 21 days. It’s not a backup of files neither an avast update. You don’t need to run it daily. It just stores parts of the executable that could be used to restoration (cleaning) in some scenarios.
Hardly, as the computer would be idle, theoretically there would be nothing to slow. However, there is a proviso to that, as idle means no mouse movement I believe, I don’t know the delay, but you could just be reading something at the time. I don’t know if that also has a CPU check also so there is nothing running in the background.
Personally I set mine to screen saver (I don’t use one, so it would never start) and do a manual Generate Now every three weeks as part of my regular weekly system maintenance. That way I’m in control over when it starts.
Remember it only kicks off every 21 days, first time out is is best to select Generate Now when you are going to be away from the computer for a while.
I don’t know if disabling it would have any unforeseen circumstances, e.g. the Repair function if available as an option uses the VRDB database so I don’t know if by disabling the VRDB if the Repair option would be offered, I just don’t know.
If you don’t use a screen saver (I don’t see the point if I’m not there) the VRDB would effectively still be enabled but you would need to remember to do the scan periodically (every three weeks).
Its generation on IDLE here has never been shown to have any impact on system speed.
This is not a particularly fast system, being a laptop, however I do leave it on frequently
when I am not using it. So I rarely see the VRDB working anyway.