After deleting did you compact all folders to remove all traces of 'marked as deleted' mail ? File > Compact folders

Are you using an Anti-virus program to scan all incoming mail, because this can slow the download and TB has no control over another program? You could switch off the email scanning.

If you use an IMAP mail account: All subscribed folders will download headers only and then when you select the email, it has to be retrieved from the server and stored in a temp cache. If you have email scanning then it will be checking everytime.

If you Synchronise subscribed folders then full copy of the folder will be downloaded to an mbox file in your Profile folders. This folder will constantly keep synchronising to update TB and also any modification to the file done in TB will need to update the server. They are one and the same folders. You can set it to only synchronise the most recent eg: 30 days. See more info here:

• https://support.mozilla.org/en-US/kb/imap-synchronization

All valid suggestions, but unfortunately in this case they have no effect. This also occurs with mail in the inbox that is no longer on the server (I have begun checking the mail on the server itself and only downloading the mail I want, just to save time), and I did compact the folders. Also, this is on a POP server. Thank you for the suggestions, though.

See https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues

Thank you Matt. I had disabled the Thunderbird extension previously, but was unaware of the additional attachment scanning. Once I disabled that, it looks like everything is working as it should. I will continue with the testing to ensure it is resolved, but right now I would say it looks good.