I thought about it for a while and realized it would be a good idea to find out where the time is lost during the display of the images as thumbnails.
I used Time::HiRes to find out.

I am actually loading jpeg data from a tar archive. I have to convert this to a Pixpuf, because Gtk2-Canvas needs a pixbuf to show the data.
The conversion function into a pixbuf consumes the majority (~70%) of the time.

This may be a hint that this won't be faster using Imlib2?