Do you have Anti-Virus scanning Thunderbird folders on startup or/and scanning emails as they are downloading? If yes, then I suggest you create exceptions not to scan any Thunderbird folder. Stop scanning incoming and outgoing mail. Attachments are opened using appropriate software and so will be scanned. All remote content which may contain malware is auto disabled by default. Are you using IMAP which has to go through a load of synchronising of folders? You could select the option to only synchronise eg: xx days. * https://support.mozilla.org/en-US/kb/imap-synchronization#w_configuring-synchronization-and-disk-space-usage