The delay could be caused by the Anti-Virus product scanning incoming mail or when opening files. The best method testing this: * Exit Thunderbird * REstart computer in 'Safe Mode with Networking' mode * Start Thunderbird. * Test opening of emails. If problem now seems ok. Make Thunderbird profile folders exempt from scanning and if possible stop scanning incoming mail. The delay may be caused because you have 'Display Attachments Inline' selected and the email in question has eg: image attachment of large size. Menu app icon > View > Uncheck 'Display Attachments Inline' (Anti-Virus scanning will also effect same process). Do you notice this only occurs with emails that have attachments? Is the problem occuring in a POP or IMAP mail account ? If IMAP, do you download full copies or only headers ? If only headers, then email is not downloaded for viewing until you select it - this saves space on computer. There are pros and cons. https://support.mozilla.org/en-US/kb/imap-synchronization#w_synchronization Do you sort and organise emails into various folders, so that no folder becomes too large? If no, you only have a few folders or keep lots of emails in Inbox, then I recommend you create some folders and organise emails and Archive old emails. Do you frequently compact folders especially Inbox, Draft and Junk/Spam to remove all the hidden data of moved emails ? If no then I advise you need to start compacting folders before a problem arises. Everytime you move or delete an email, the original email is still in the original folder - it is hidden and 'marked as deleted', but it is not properly removed until the folder is compacted. So the file could be a lot bigger than you would expect.