I use the picture tag with a bunch of media queries to deliver optimized images for each resolution in websites that I build, resizing a 1080p image to only 200px width does wonders to mobile performance while keeping it perfect for full HD monitors.
Since Zola has an image resizing feature and shortcode-snippets, this could be a nice way to automate things away (i‘d hate to slice pictures for X sizes by hand).
Your pages are excellent by comparison to comparable offerings of similar information density mostly seen.
But there's always room for experimentation.
How about preserving a copy of your portfolio page now (and the PNG files it's now using) and giving it an address like /portfolioOLD?
Then using an image editor, ruthlessly resize/resample-at-lower-bit-depth one of your PNG's so their actual rectangular pixel dimensions are about the same size that they appear on a full-size monitor now.
Then ruthlessly compress it until it looks just a little less high-quality than it does now. Just a little bit, you want to be able to tell the difference but you don't want other people to notice. These are just thumbnails anyway.
Use these editor settings on the rest of the PNGs, renaming them accordingly as you go.
Deploy the new portfolio page linking to the resized renamed thumbnails instead.
Just guessing, but I expect it can bring the load time down to about 10 percent of the old portfolio.
And it would be really easy for anyone to A/B test and get representative numbers.
