http://bugs.winehq.org/show_bug.cgi?id=22684
--- Comment #4 from Roderick Colenbrander thunderbird2k@gmail.com 2010-05-16 16:56:54 --- Created an attachment (id=28027) --> (http://bugs.winehq.org/attachment.cgi?id=28027) RealizePalette front buffer patch
Could you check if this helps? The patch disables a certain path in RealizePalette. I'm not sure sure whether this path makes sense for the back buffer since it might unneeded downloads/uploads (I mean the contents of the framebuffer might not contain the new frame yet but on the other hand you don't know this before hand; it might also just be called when the frame is done).
Anyway just try the patch. If it works I might have a better idea where to look.