Compare First Contentful Paint (FCP) between optimized (inline CSS) and non-optimized (external CSS) pages.
Optimized (Inline CSS)
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Non-Optimized (External CSS)
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How to test: Open DevTools, go to Network tab, enable throttling (3G or Slow 4G), disable cache, and click "Run Test".
Note: If your server uses HTTP/3 (QUIC), the difference may be less pronounced due to 0-RTT connection resumption. For the clearest results, test with HTTP/1.1 or HTTP/2, or use a fresh incognito window.