Windows 7, as you know, is a beta version. It can have all kinds of problems. Therefor Windows 7 is not in the list of supported operating systems by avast. Yes, the network shield can make browsing slower. “Play” around with the settings for it and see if it makes a difference.
PS:
Can you run HijackThis and sent me the log file?
I have a little util that can analyze logs from HJT and want to update it so it will also recognize Windows 7.