If it is a true scheduled boot scan, e.g. before windows starts, then what Igor said is applicable.
Or are you talking about overall duration every boot time is slower ?
If so what is the Sensitivity setting for Standard Shield ?
Normal is the default, the higher the setting the more files will be scanned.
What other security based software do you have that might be running at boot, e.g. Spybot S&D (TeaTimer), AdAware (AdWatch), SpySweeper, PrevX, WinPatrol, ProcessGuard, etc. ?