Using Thunar 1.2.1 on Solaris x86 Thunar does an excellent job of generating thumbnails for files. Sensibly, if you're browsing a huge directory, it only generates thumbnails for the files that are visible. One nice enhancement would be to generate thumbnails in the background for the files that can't currently be seen, so that they appear instantly when you scroll to them rather than having to wait for them to be generated. You don't want to do the whole lot; perhaps one "page" worth of scrolling would be worthwhile. (That way, the common usage pattern of simply scrolling or paging through a directory would appear much smoother, with the thumbnailer having a chance to be one step ahead.)
Render as you see it is much better resource wise.