I think this is just a fairly simple site that's actually quite fast by default!
Browsers are actually incredibly well optimised to load these types of simpler pages with mostly just HTML, CSS, and a sprinkling of JavaScript.
Where they have to do more work (and where PageSpeed Insight's reduces scores) is when you add a lot of JavaScript that "gets in it's way" to load the page. Images themselves can cause performance issues if they are too big, but mostly the browsers can handle it. In particular on this site 1) these don't seem two bad (apart from two detailed images that would likely only be able to be optimised so much) and 2) they are lazy loaded (with loading=lazy attribute) so they don't cause issues. That's not to say you can't optimise these more, but I don't think there's huge gains in there to be honest. At least by the metrics PageSpeed Insights measures.
Keep up the good work (or lack there of by staying out of the browsers way 🙂 )
Thanks,
Barry