Binary package hint: nautilus
If I open a directory with many image or movie files for the first time, you can see nautilus generating the thumbnail previews for that directory because they appear slow enough for you to see it. When you visit the directory a second time, those thumbnails are cached and stored locally so they do not need to be regenerated and they appear instantly.
If, however, you create a symbolic link to that directory and then visit it through the symbolic link, the thumbnails will be once again regenerated. This is visible because they appear slowly and CPU usage goes up as if you were opening the directory for the first time. You can do this repeatedly for any number of symbolic links and it will regenerate them each time.
Nautilus should recognise the destination directory of the link and use the cached versions instead of regenerating them each time.
Thanks for the report. While valid, this bug which should rather be taken upstream.