Images, Fonts, Third-Party Scripts: LCP and CLS

Images, fonts, and third-party scripts are the three categories responsible for the most field LCP and CLS regressions. They interact: a 12KB GTM tag firing synchronously in blocks the parser for 200ms, which means your hero image (already the LCP candidate) now loads 200ms later. LCP jumps from 1.8s to 4.2s. CrUX data reflects it three weeks later. What this covers: fetchpriority and for LCP images, WebP vs AVIF trade-offs, srcset / sizes resolution selection, font-display values and what each