You know that avast has a built in help, right?
The shields can be paused from the tray icon.
For avast to be "delayed" on startup, it's a little bit more complicated (it is not in the main GUI of avast), and there shouldn't be any real need for that anyway. Simply pause the shields for 10 minutes from the tray icon and reactivate them when you want them again (also from the tray icon).