The option shown by avatar2005 only removes the memory scan prior to the Start avast! antivirus, before the Simple User Interface opens and not a scan at boot.
I think you are mistaken with the schedule boot-time scan option (only for the next boot and not before every boot), which is a long scan and the scanning of running files on boot. With my avast on default settings , the standard shield on Normal, there are only about 100 files scanned on starting my system and no major delay.