What other security based software do you have that might have an effect, e.g. scanning during boot this causes avast to also scan the files it opens: SpySweeper, Spyware Doctor, PrevX, WinPatrol, etc. ?
Firewalls can also have an effect, mine, outpost pro has an anti-spyware plug-in (disabled in mine) which is very active at boot. This caused an extra 1000+ files to be scanned by avast, two things helped, 1) disabled the plug-in and reduced the Standard Shield sensitivity back to Normal, the default.
I'm still not sure if this is for your home system or if your connections is dial-up (or if your are using that as a means of stopping the check) or broadband.
In either case would delaying the check improve things, by editing the avast4.ini [InetWD] section and adding or editing this line (for broadband connections) AlwaysConnectedWaitSeconds=60, I think the default is 30 sec or (for dial-up connections) RASWaitSeconds=120, the default is 20 sec.
[InetWD]
RASWaitSeconds=120 ( Or )
AlwaysConnectedWaitSeconds=60 ( Only one would be applicable )