http://lists.roundcube.net/mail-archive/users/2010-07/0000021.html
I'm not sure all of you are reading users list. THREAD preformance issue has been fixed. Still on big mailboxes it can took a few seconds for handling threads. BTW, my proposition to make displaying messages (in 'show' action) faster is to create page navigation (#countcontrols) in separate (ajax) request after message.html page is loaded. Any thoughts?