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.