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I have moved quite a chunk of emails into local folders and it's still slow. (There are about 200 on one address, 400 on the others.)
To my mind 200 and 400 emails are almost trivial numbers, I don't think that's causing your speed problem.
Just wondering something. On Windows* (at least this was the case decades ago) if you uninstalled then reinstalled a misbehaving program all the settings stayed in place. Does the same work with an email client on Linux? And is that even worth trying, more to the point?
Bev
*sorry.