I really think your issue is probably antivirus. (among other things safe mode stops them injecting their spam/scam code into Thunderbird while it is running). Try inserting an exception in the antivirus for the Thunderbird profile folders to prevent scanning while the program is running. Do you have a remote calendar? Does it refresh and slow things? One of the things that has occurred is moving messages in IMAP accounts is now done sequentially, one after the other, not as a block as there were ongoing issues with mail simply being lost dues to timeouts when it was done as a block of mail. As this requires a round trip to the server for each email involved, anything that slows the connection can be a problem (again think antivirus and try running without it) Other things that could be relevant is the health of the disk that the data is stored on. Have you checked it with operating system tools? It may appear out of left field, but the mail files are some of the most frequently updated and therefore also ones that tend to be fragmented. If they end up in a location where reading is either broken or slowed by pending failure then everything essentially drops to the same access speed. Windows defrag can be helpful with this as well and chkdsk. When it comes to sending, there is an expectation that it will be relatively slow as all of the sending is done in the foreground. Most mail programs just record your request and actually send the mail "later". Thunderbird contacts the server and sends the email and gets an acknowledgement from the server that everything is Ok before it tears down the sending dialog. Slower from the user perspective, but error reporting is done in the immediate send, rather than some delayed dialog. You can force send in background. There is a discussion on that here https://support.mozilla.org/bm/questions/1358161 just be aware error reporting is the reason it is not enabled by default.