For months my Thunderbird install on two PCs has been an absolute dog, with it constantly freezing and the titlebar becoming (Not Responding) whenever I do anything. I mean anything, from simply clicking an email window, to selecting an email and it trying to load the preview pane. Selecting 10 emails and hitting delete would provoke a delay of about 20 seconds. Moving an email from one folder to another was pretty much impossible. Very disruptive and frustrating given how much time I spend in email every day. Anyway, as I said, months have gone by. There's a big FAQ on Mozilla's site with various things to try, which I worked through. Many other people have this issue in Google but nothing seemed to work. Also, it's happening on two PCs, one Windows 10, one Windows 7 Pro, so I figured it must be something to do with gmail's innards or something off-PC. So I just kinda dealt with it, given I have zillions of emails going back decades, shit's gonna be slow. Today I found yet another thing, but this actually worked: adding the Thunderbird profile directory to the excluded list in Windows Security Essentials. I'm back to snappy now, on the Windows 7 Pro PC anyway. I haven't made the change on the Windows 10 PC yet. But on this PC everything is instant again, as soon as I made that config change. Didn't even restart Thunderbird. Select 10 emails, delete, instant. Move 3 emails to another folder, instant. Click away and back to Tbird, responsive. Fookin' yay. Anyway, hopefully this is of use, just had to get it off my chest and share my joy.