In cssu-devel repo is updated modest with patch which postpone recurive operations (imap folder tree) via glib main loop. This can reduce using (100%) cpu by modest (and also reduce height of recursion). But can cause that modest will be slower... Please test how modest working now. Specialy on imap accounts with lot of imap folders (~= 50) with lot of emails (~= 50000) and with enabled offline imap mode.