Google is revising and reversing some of their rules of Google Page Experience. New rules are supposed to be implemented in May. So, you may want to wait until then to make your own changes (if you care).
For more detailed info (and a related info), read this article and the article it links to.
Page load times do matter, and they are a ranking signal for Google. I can get pretty great scores with Foundation 6.
In may, Google will assign a small ranking boost to sites that pass all core web vitals. Do you want to pass them? Yes. Are people being a little alarmist about impact? Also yes.
Fast page scores are possible if you compress images, have efficient caching, locally host fonts, lazy load images and iframes, and preload, prefetch, and preconnect resources where you need them.
It’s good to consider this stuff in Rapidweaver just as any other platform, be it WordPress or something else.
Here is a page that takes 10’s of seconds to load but scores 100 on Lighthouse and Page Speed. Tools to measure performance don’t always get it right, screwing with JS can get great scores even if the page doesn’t really load that fast.
Here is quite good explanation of what Google is after in their new rules related to Core Web Vitals, coming in May, with additional future prognostics on the same topic (again: if you care).