CLS & LCP Google Web Vital Concerns (PHPBB)

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Looks like Google Search console is starting to penalize my sites for poor Mobile and Desktop core vitals. It's not happening for all forums. One most impacted is specifically for:

CLS issue: more than 0.25 (mobile)
Poor LCP issue: longer than 4s (mobile)
CLS issue: more than 0.25 (desktop)

In the past, this seemed to be an issue in search console then disappear - then re-appear sometime later. As of the last few months, seems more and newer links are getting the LCP and CLS issue.

Google:

LCP (largest contentful paint): The amount of time to render the largest content element visible in the viewport, from when the user requests the URL. The largest element is typically an image or video, or perhaps a large block-level text element. This is important because it tells the reader that the URL is actually loading.

CLS (Cumulative Layout Shift): CLS measures the sum total of all individual layout shift scores for every unexpected layout shift that occurs during the entire lifespan of the page. The score is zero to any positive number, where zero means no shifting and the larger the number, the more layout shift on the page. This is important because having pages elements shift while a user is trying to interact with it is a bad user experience. If you can't seem to find the reason for a high value, try interacting with the page to see how that affects the score.

And...

My site status changed, but I didn't change anything
If you didn't make any changes in your site, but you see a big change in status for a lot of pages, it's possible that you had a borderline status for many pages, and some site-wide event pushed your pages over the edge: for example, your site traffic dramatically increased or the service that serves your image files experienced a latency change, either of which could slow your site down. A small, but site-wide, change might have been just enough to push a bunch of borderline Good pages into the Needs improvement category, or from Needs improvement to Poor.