I don't know what to suggest about any start-up lag other than to monitor the other posts you say you have seen as I haven't seen any such lag and my system is no speed freak.
How do you connect to the internet, broadband, dial-up, how ?
If broadband, it could be that your connection is established early and avast detects that and checks for an update, if that is the case then it may be an idea to delay that check for a few minutes. It is possible that how to do that is outlined in the other topics, but I will repaet it here.
You need to edit the [InetWD] section of the C:\Program Files\Alwil Software\Avast4\Data\avast4.ini file and add the following line (depending on your connection type enter one or other of the Bold lines below):
Dial-up connections, add this line:
RASWaitSeconds=120Broadband connections, add this line:
AlwaysConnectedWaitSeconds=120[InetWD]
AlwaysConnectedWaitSeconds=120
Or
RASWaitSeconds=120
The figure is seconds and the above equates to two minutes, you could try that and adjust upwards if required, 180, 240, etc. It may be worth your while extending that delay to 240 or 300 seconds, so you can be sure there would be no possible interaction with the update check and completion of boot.