When thunderbird donloading new messages at main window also other functions as new message writing lags badly. It is understandable that it will slow down some of features but it is very annoying that when you write message characters come with 1-3 second delay. I have i7-4770K computer with 32GB memory. Also when thunderbird download new messages there is plenty of free cpu. So it looks more like some kind locking problem between threads. Even new message is at new window it still lags. I use windows defender and it will slow down message downloading (if there is hundreds of messages to load) so some times it is long wait that software come back to usable. Way to reproduce this problem is that install thunderbird to windows 10. Use windows defender. Sync new account or make new folder and transfer 100-500 messages to it. After some while thunderbird start download moved messages and now if you try write new message that textbox is really laggy.