Hi sipx,
in Version 1.6 of the Portfolio, we added a thumbnail generation system, now, thumbnails are generated automatically which should improve loading time of the Portfolio.
Unfortunately, the width/height values for the thumbnails are hardcoded presently, the respective configuration setting will be added in the next version.
As you correctly stated, V1.5 displays the images in the original ratio - so you should use this version until we add the config option.