I had the same problem. I fixed it by replacing the code
jQuery( img ).bind('load', function() { appendImage(img, num); });
with
appendImage(img, num);
in files wp-content/plugins/nextgen-gallery/js/ngg.slideshow.js and wp-content/plugins/nextgen-gallery/js/ngg.slideshow.min.js
This fix has the drawback that it won’t wait until the first image has completely loaded when it starts to load the next one.
That might cause a problem on some really slow web servers or with very big images but worked fine for me.
