One of the most common reasons for this is an Anti-Virus because it's scanning incoming files and also maybe scanning any file that gets opened and updated with new mail. Files get downloaded and then appended to relevant file eg: Inbox mbox file. Advise: make your thunderbird profile folder and it's contents is exempt from being scanning. Also switch off scanning of incoming files/emails - even if this is just for a day or two to test performance. One quick way to ensure a proper test is to do the following so AV and other similar types of programs are temp disabled without the need to alter anything: Exit all programs Restart computer in 'safe mode with networking' - note - if currently wireless using WIFI then you will need to connect computer to router/hub using an ethernet cable. Then start Thunderbird and see if things improve. Another main reason for delays are files containing emails which are getting rather large, so take up memory etc when they get opened to write data into. If you have a lot of emails in some folders then suggest you create some new folders with suitable names for organising and storing mail. Then compact folders. I would advise you manually compact certain folders on a regular basis. Any folder that receives mail which is subsequently moved or deleted will still have a copy of email in original folder but it is 'marked as delted' and hidden from view. Compacting means a new file is created and then it looks in the original file for anything 'marked as deleted' - it does not copy those emails - only all the good emails you want to keep are copied into that new file. Then original is removed and new file replaces it. The new version is like a cleaned up file. It's considered good housekeeping maintenance and helps to reduce file size, recover disk space and reduce the risk of corrupt file. The three folders that usually need compacting the most are 'Inbox', 'Junk/Spam' and 'Drafts'. But also any folder which you know you have deleted or moved a load of emails. To compact: Right click on folder and select 'Compact'