It's kind of hard to fix some of these Web Vitals issues when I'm struggling to actually believe they even exist...

As far as I'm concerned my pages are lightning fast: bespoke design, lightweight CSS, webp images, lazy loading, deferred JS, caching and global CDN. Yet I check on PageSpeed Insights and it tells me I have an LCP of 3.2 seconds - and that this is the first block of text on the page?!
Looking at the thumbnail sequence, the page is pretty much rendered in the first one...

On the mobile version there are no images or ads above the fold.

When I check in Chrome's DevTools it shows LCP at around 300ms.