By all accounts you need to regularly compress your email folders in Thunderbird as deleted files aren’t actually removed (just the reference to them) until you compress the folders.
Sorry I’m not a thunderbird user, so I don’t know how you compress the folders or if this is the correct terminology.
Is the Standard Shield’s sensitivity set to High level?
You can add TB files to the Exclusion lists of the Standard Shield provider if you’re using the Internet Mail provider of avast. Something like:
c:\Documents and Settings\Administrator\Application Data\Thunderbird*
c:\Program Files\Mozilla Thunderbird*
But, remember, this is not a solution, just a workaround.
Just a bit puzzled, are you hinting that there is a problem with Thunderbird and slow mail checking? Certainly there is on my PC, but is this more general, is it linked to Avast?
The work around you suggest doesn’t it leave the mail folders vulnerable to viruses?