site loads very slowly

My travel blog is loading very slowly, especially the photo galleries and lightbox. can anyone take a look and make specific suggestions? Or recommend somewhere I can go for such advice?

I’m into photography, so I want to have lots of high quality images (and retina, when possible). But I don’t know how to optimize them to load reasonably fast while maintaining very high quality. Or it could be something else causing the slow load times. I really don’t know.

A sample gallery page: https://theslowroad.org/wp/galleries/europe/belgium/ghent-at-night/