Optimizing For Bandwidth

PageSpeed's default RewriteLevel of CoreFilters is designed to reduce latency, incurring a small risk of page breakage. A related goal, bandwidth reduction, can be achieved with close to zero risk of web-site breakage:

Apache:

ModPagespeedRewriteLevel OptimizeForBandwidth

Nginx:

pagespeed RewriteLevel OptimizeForBandwidth;

This option is suitable for use in a root configuration at a hosting service, CDN, or multi-site setup, in conjunction with InheritVhostConfig. In this mode, PageSpeed does not alter HTML at all. It compresses and transcodes images in place, and minifies JavaScript and CSS. By avoiding changes to URL syntax and to HTML, the potential problem of user-written JavaScript encountering unexpected DOM elements is elimimated. There is still latency benefit due to the reduced size of resources, as well as substantial bandwidth reduction.

Bandwidth Savings

The bandwidth savings resulting from this option are, of course, dependent on the source site. Our tests with the default settings for OptimizeForBandwidth indicate an average bandwidth reduction of roughly 37% over the content of 1000 large web sites with a client that is capable of displaying webp images, and 25% improvement with older clients.

Adding Additional Options

Additional latency optimizations that modify HTML can be layered on top of OptimizeForBandwidth by enabling additional filters, and altering the Preserve URL settings for CSS, images, or JavaScript. For example, to set up a shared hosting server with OptimizeForBandwidth in the root, but where individual sites have added inline_javascript, lazyload_images and prioritize_critical_css, or simply turned on CoreFilters, the following pagespeed.conf fragment may be used:

Apache:

ModPagespeedInheritVHostConfig on
ModPagespeedRewriteLevel OptimizeForBandwidth
<VirtualHost *:80>
  ServerName prioritize_above_the_fold.example.com
  ModPagespeedEnableFilters inline_javascript,prioritize_critical_css,inline_preview_images
</VirtualHost>
<VirtualHost *:80>
  ServerName preserve_css_urls_off.example.com
  ModPagespeedCssPreserveURLs off
</Directory>
<VirtualHost *:80>
  ServerName core.example.com
  ModPagespeedRewriteLevel CoreFilters
</Directory>

Nginx:

pagespeed RewriteLevel OptimizeForBandwidth;
server {
  server_name prioritize_above_the_fold.example.com
  pagespeed EnableFilters inline_javascript,prioritize_critical_css,inline_preview_images
}
server {
  server_name preserve_css_urls_off.example.com
  pagespeed CssPreserveURLs off
}
server {
  server_name core.example.com
  pagespeed RewriteLevel CoreFilters
}

In version 1.9.32.1 and later, In Place Resource Optimization is enabled by default. In earlier versions, turning on OptimizeForBandwidth automatically enables In Place Resource Optimization to optimize the resources without needing to change their URLs. In these earlier versions, when a VirtualHost overrides the RewriteLevel to CoreFilers, that turns off In Place Resource Optimization, but it can be re-enabled explicitly:

Apache:

ModPagespeedInPlaceResourceOptimization on

Nginx:

pagespeed InPlaceResourceOptimization on;