Images sequentially loaded – slow PageSpeed

We always try to optimize the pagespeed of our sites.
Therefore we use caching-, minifying plugins and CDNs.

Just take a look at this result

Or test the URL http://haus-und-garten-wiesel.de/landschaftsbau/treppen/
on Pingdom

As you can see the site is being loaded very quickly besides of the gallery images.
They are both being loaded sequentially and with delays between the beginning of loading an image.

Why?

https://wordpress.org/plugins/flash-album-gallery/