After some further investigation we (I and OP) concluded that this issue is some kind of conflict between win7 and avast core driver.
This issue (LAN file sharing slowdown) only happens in Windows 7 (both RTM/SP1, not sure about Vista), and XP is not affected.
Local disk access is not affected either.
Even all shields are uninstalled this issue remains. Only way to avoid it is uninstall avast completely.
I confirmed this issue on my side some weeks ago, and tomorrow I'll have a chance to access suitable environment for testing. So, please give me some advices or testing procedures to determine what is wrong and find solution for this issue.
I don't want to advice uninstalling avast due to only this issue, but LAN file access is very important for business or large-scale use so we can't just stand this slowdown.
OP already sent a ticket and seems support section failed to solve this issue, so he came to this forum for help.