If the anti-spyware is incorporated into the ZA firewall it is active on boot. I had the same issue with my firewall Outpost Pro (anti-spyware plug-in) it is very active during boot, that caused avast to scan more files that it would if there was no anti-spyware running on boot.

As a suggestion, check the Standard Shield details directly after boot (when the icon stops rotating) and tell me what the scanned total: reads ?
At the same time what is the Sensitivity set to (Normal is the default), the higher that is the greater number of files scanned ?