It's very odd. Normally moving an email, say from the Inbox to the relevant year's archive, takes "no time at all".
Which is true for all the mailboxes shown on the attached screenshot, EXCEPT for the four at the bottom. Moving even one message from Inbox to Yiddish can take ten seconds or longer (with the dreaded "Not Responding" occasionally). Moving something in the opposite direction is also screwy, only from those four.
I had thought that it was because they're rather large. This first started when there were just Yiddish and MSW, and I split off most of the earlier years into the archives shown. To no avail.
It's only an annoyance, but I'm puzzled about how such a thing could happen.
