What is your standard shield sensitivity (default is Normal) ?
Increase that and you will see an increase in files scanned and boot duration.
Check the programs that are starting at boot, only allow those that are essential and have to start on boot to start. Many programs when they are installed set themselves to start on boot, media programs for instance, you only need them if you are about to open a media file.
Do you have any other security programs that run on boot ?
This can cause double scanning as avast scans the files it wants to open and scan.
What is your firewall ?
They can be quite chatty opening many files or have plug-ins that are acting in a similar way to security programs, this could also result in double scanning.