I’m wrong… there is such an option into the [MailScanner] section of avast4.ini file.
PassThrough=0
Specifies the behavior of the Mail scanner in situations when the message cannot be tested: lets the messages pass through without testing = 1; does not allow any messages to pass through until the resident task is running or paused = 0; will not allow the mail program to connect to a mail Server = any other, different from 1 or 0 (you may encounter error messages from your e-mail program).
More info, click on Settings in my signature.
Oh, speed is normal here 8)