OK, back.

First option would be to try to delay when avast! starts it’s system services. You get to keep all shields active and set the way they are.
(Forgot this was already done.)

Avast! program interface>Settings>Avast! Settings>Troubleshooting>place a check in the box for “Load avast! services after loading other system services.” Close the program interface. Reboot.

Seems some definitions are in order re the three shields.
Web Shield: Monitors all web browsing activity and proactively blocks online threats before they reach your browser. So, this one is for surfing protection.
Network Shield: Provides proactive protection on viruses on the network, and also blocks known malicious websites on-the-fly using a blacklist of known bad sites with known malicious code (hacked). So, this one would block worms that spread through the network, for example, or block hacked sites.
Behavior Shield: Monitors your system for suspicious behavior of system and application files and alerts you when such occurs. So. if a normal system file exhibits behavior it is not classified to have, it will alert. More heuristics-based detection than definition-based detection.

As long as you run a protocol where you test by disabling one shield at at time, and do not actively surf the 'net, you should be fine. Reboot after re-enabling each disabled shield to ensure they are running at next restart, and then repeat the same process for the next one.

Keep in mind that when a system starts a cold boot or restarts, there is a lot going on “under the hood” one cannot see, and it is happening pretty much all at once. Sometimes services/drivers will not load when they are supposed to, and consequently will temporarily hang until they start properly, and thus slow expected overall boot time/performance over what it should be. This may be what you are seeing.