Large folders? How big? What type of folders? Have you seen https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems ?

I have seen the link that you mention. But even in Troubleshoot mode, it is slow with the only 1 add on disabled. The folders are not big at all. Just IMAP. What seems to be very slow is the expand button, which even for 1 level goes terribly slow.

I am now using 115.3.3. Thunderbird has been slow starting from 115.x.x. I can casually count to 20 after starting it till the folder and email lists are displayed. I have 8 email accounts using IMAP and downloading headers only. The previous version will appear after counting about 3. Same when the anti-virus is disabled. This is a comparison using the same computer, so anti-virus, add-ons, memory, etc. are all equal. This problem appears on my desktop computer, my notebook computer and my colleagues'. They therefore have removed 115.x.x and re-installed the previous version. Only I keep on with 115.x.x.

Exactly as you describe it @kctang. However in my case I can't go back to any older version as it has also converted my profile. It is a pity that there is such a big performance impact.

Again, size information previously requested will be helpful.

I moved to Safe Mode as suggested, great, it opened message straight away, but despite asking for networking it didn't so I had to restart. Now what? Safe works but......