The behavior in the material room is interesting. When I make a change, all the previews go black, and then show up one by one. It happens fairly quickly, faster than before, but it does seem like it is redoing some previews for nodes that didn't get any changes (even indirectly in upstream nodes). If it was a little smarter about which ones to update, that could speed things up even more. Also makes me wonder whether the whole thing would run faster with all the preview images closed.