Static assets have appropriate Cache-Control headers

Static assets served without long-lived Cache-Control headers force every returning visitor to re-download CSS, JavaScript, and image files they already have — identical bytes transferred repeatedly for no reason. A 500KB bundle re-downloaded on every visit costs 500KB × daily_active_users in bandwidth per day. With Cache-Control: public, max-age=31536000, immutable, that same bundle is downloaded once per user per year. For users on metered connections or high-latency networks, the cache miss is also the difference between a 500ms and a 3-second load. ISO 25010 performance-efficiency.resource-utilization flags repeat downloads of identical resources as a direct waste.

Configure Cache-Control: public, max-age=31536000, immutable for all hashed static assets, and max-age=0, must-revalidate for HTML files. On Vercel and Netlify, Next.js applies these headers to hashed chunks automatically — verify no override is negating the defaults.

// vercel.json — explicit headers for custom infrastructure
{
  "headers": [
    {
      "source": "/_next/static/(.*)",
      "headers": [
        { "key": "Cache-Control", "value": "public, max-age=31536000, immutable" }
      ]
    },
    {
      "source": "/(.*)\\.(js|css|woff2|png|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: app.use('/static', express.static('public', { maxAge: '1y', immutable: true }))