Open a window with a gazillion photos. 
Navigate to a subfolder that also have a few photos.
Watch the thumbnails take forever to appear because all the gazillion photos
in the parent dir are ahead in the queue.

Maybe we should prioritize the currently open folders so that they get
their thumbnails before non-active ones? Also, I dont know if it is
possible to tell which window is having the focus, one could thumbnail that
one first too if possible

It is not a big issue with small files, but with digital photos (which
nautilus seems to be very popular tool to organize with) make this an
issue, thumbnailing a 2MB jpeg takes a while.