I'm all for them killing it.
Why would it be slow? I can only type so fast and sending/receiving mail is mostly dependent on the connectivity, not on the software.
I have a need for more people to repeat the same process. There's zero chance I'll be asking them to download TB.
This means depending on your storage configuration and how you use folders, potentially writing 100MB, 1G, 10G, etc. back to disk on folder content change.
For batch processing though, I use the CLI tools - thunderbird is view/compose only (procmail/formail/archivemail/offlineimap,etc)
I was hitting performance probs on bigger folders, and setup some more strict archiving to archive older mails to subfolders/zap offline using an 'archivemail' job, etc, and the problem went away, with TB being quite snappy.
as I understand it there was some effort to support 1-file-per-email, which might be an option now, but this has it's problems too.
Outlook has it's PST corruption hell...
Basically, in my opinion, email is a pain, each client shows it differently, though I am admittedly not up to speed on Opera Mail.
You seem to be attempting to address just the slowness of my complain and not its overall awfulness. Start-up time, loading each email; I've never seen it work very fast it always has that android-like lag throughout the entire thing. It also doesn't feel native to whatever OS you're using it in almost like using a Swing app.
The UX is just awful.
So if you think Thunderbird should be killed, what do you suggest as a replacement? It needs to be free, and run on Windows for my company to use it.
I suppose I'll have to learn to live with mutt eventually.
Mutt is the only way I can keep up with my mail volume. The only time the UI doesn't react instantly is when it loads huge mailboxes. And it still does that faster than I've seen graphical clients load the same mailbox.
Add to that pretty much infinite customization, and it does indeed suck less.
If you have to deal with large amounts of mail for pretty much whatever reason, you can do it faster with mutt than with any other client I've ever used. Yes, there is a learning curve. It isn't a vim-style learning curve, but it is there. But if you're considering a command line MUA at all, you can handle it.
There's a bunch of config tweaks that are supposed to help but so far they haven't.
Also, sometimes it'll just not find certain messages. That also means it's time to clear out and rebuild indexes.
This has been my experience every time I use thunderbird, over many years. I'm not sure what it is about my mailbox that makes me see these issues when others don't.
It's a good program but could use some maintenance loving.
If you like thunderbird theres postbox developed from thunderbird