Static assets have long-lived cache headers configured

Browser cache headers control whether a returning visitor re-downloads assets on every page load or serves them from the local disk cache in milliseconds. Without Cache-Control: public, max-age=31536000, immutable on hashed static assets, every visit — including from users who visited yesterday — initiates fresh network requests for all JavaScript bundles, CSS, and images. ISO-25010 resource-utilisation covers this: downloading the same unchanged 200KB bundle on every repeat visit is pure waste. Modern frameworks (Next.js, Vite, webpack) include content hashes in filenames, making year-long browser caches safe — changing any file produces a new filename, so stale cache never serves outdated content.

Configure long-lived immutable cache headers for hashed static assets and short-lived revalidation headers for HTML. On Vercel with Next.js, static chunks already receive max-age=31536000, immutable — verify this is not overridden.

// vercel.json — explicit rules for custom asset types
{
  "headers": [
    {
      "source": "/(.*)\\.(js|css|woff2|webp|avif)$",
      "headers": [
        { "key": "Cache-Control", "value": "public, max-age=31536000, immutable" }
      ]
    },
    {
      "source": "/(.*)\\.html$",
      "headers": [
        { "key": "Cache-Control", "value": "public, max-age=0, must-revalidate" }
      ]
    }
  ]
}

For custom Express servers, add serve-static with maxAge: '1y' for the public/ directory, or set headers explicitly in middleware for the /_next/static/ path.