Problematic Load times

I am experiencing excessively slow load times in this otherwise amazing theme. I think the issue is that the theme isn’t loading thumbnails but, rather, the full image.

I tried using this in my child theme’s style.css

div.preloader {
display: none;
}

But it disabled the responsive grid, breaking the theme. (I don’t know if I added it in the wrong place in the css file, though I tried several different places to no avail.)

I had hoped that the new version of WordPress (4.5) would help with this issue, but it is the same as it was in the previous version.

I would sincerely appreciate help with this!

You can look at the load times at my site:
https://maternitygallery.com/

And you can repeat those load times by selecting any of the categories and letting it reload.

Thanks!