Avast scans only when opening a mail, not the whole folder?!?!
I won't be so sure as the email folder is a packed archive.
avast! or any other antivirus will scan the pop3/smtp traffic AND the Standard Shield will scan the whole file (specially if you're using the High sensitivity level).
In Inbox normally max 100 mails...
BTW: I meant 2-3 seconds not minutes (plain text email)
Well, I meant minutes if you're keeping all that number of emails in TB.
By the way, take a lot of care: if ANY email comes infected and when warned by avast! you delete it, ALL YOUR INBOX will be deleted.
Search the board and you'll find this TB limitation (in TB forums you'll find the same).
Do you use any spam killer?
Are these emails with attached files?
Are you using the IMAP protocol?