My Gmail account has been too slow lately. As it approached 90% size, I tried to reduce its size, by deleting old mailing list emails.
I deleted 3 times more than ~50 k emails, and each time, my account became unavailable for several minutes (about 30), including through IMAP! Got several errors on the web page (16 or 17 and 67 if I recall properly).
You've been warned. Plan ahead if you try to do the same thing :)
Final words: I am now down to 57% gmail usage. Need to find or write a tool to be able to identify the culprits. Maybe some kind of FUSE IMAP file system to reuse something like baobab (aka http://windirstat.info/ for Gnome) ?