Pre-rendering Optimization

April 20, 2026 · View on GitHub

Contents

Speed up rendering

Tell the pre-rendering engine when the page is fully rendered via the window.IS_RENDERED global variable. Define it as false at the very beginning of your page or application bundle script — ideally inside the <head> tag:

<html>
  <head>
    <script type="text/javascript">
      window.IS_RENDERED = false;
    </script>
  </head>
</html>

After the page is fully rendered and populated with data, set IS_RENDERED to true:

<html>
  <head>
    <script type="text/javascript">
      window.IS_RENDERED = false;
    </script>
  </head>
  <body>
    <script type="text/javascript">
      asyncFunc(function (err, res) {
        // More synchronous code here
        window.IS_RENDERED = true;
      });
    </script>
  </body>
</html>

It is a good idea to set a safety timeout in case the page never finishes rendering (due to an error or bad application logic):

setTimeout(function () {
  window.IS_RENDERED = true;
}, 6000);

Nginx tuning

If the default Nginx integration settings do not satisfy your use case, tune with the following directives.

Place proxy_cache_path in the http context, and the location-scoped directives inside your existing location @prerendering block.

# http context
proxy_cache_path /var/lib/nginx/cache levels=1:2 keys_zone=backcache:10m max_size=5G inactive=60m use_temp_path=off;

# inside server { ... } block
location @prerendering {
  # Enlarge these directives in case of 408 errors
  proxy_read_timeout    25s;
  proxy_send_timeout    35s;
  proxy_connect_timeout 45s;

  # Disable buffering
  proxy_request_buffering off;
  proxy_buffering         off;
  proxy_buffers           256 8k;
  proxy_buffer_size       8k;
  proxy_busy_buffers_size 64k;

  # Enable caching
  proxy_cache            backcache;
  proxy_cache_use_stale  error timeout http_404 http_500 http_502 http_503 http_504;
  proxy_cache_key        "$scheme$request_method$host$request_uri";
  proxy_cache_valid      200 301 302 10m;
  proxy_cache_valid      404 2m;
  proxy_cache_valid      any 1m;
  proxy_cache_lock       on;
  proxy_cache_revalidate on;
  proxy_cache_bypass     $http_pragma $http_upgrade;
  proxy_cache_min_uses   2;

  # Cache for descriptors
  open_file_cache          max=1024 inactive=12h;
  open_file_cache_valid    12h;
  open_file_cache_min_uses 2;
  open_file_cache_errors   off;
}

Further reading