Japanese Web Fonts and Core Web VitalsFixing LCP and CLS by Measurement
· 12 min read
When I audited this site's (notes.rewheel.dev) SEO, the technical items were almost a perfect score. The sitemap, robots.txt, canonical, OGP, and JSON-LD were all in place, and the Lighthouse SEO score was 100 on every page. Yet only Performance was stuck at 58-63.
As I dug in, what was dragging it down were two of the Core Web Vitals — LCP (loading speed) and CLS (layout shift). And the culprits were in different places than I first suspected. LCP was how the Japanese Web font was loaded; CLS was not the body images but the header logo. This article is a record of pinpointing those culprits by measurement and fixing them without adding measures that do not work.