In addition to the rewriters explicitly selected, PageSpeed Service provides the following capabilities by default.
Gzip
Content served by PageSpeed Service is compressed using gzip to reduce the bytes sent over wire, especially for HTML, text, CSS and JavaScript. We also selectively compress responses only for those user agents that are known to support Gzip, to ensure it works correctly on all user agents. This feature implements the PageSpeed rule for minimizing payload size.
HTML cleanup
PageSpeed Service removes unnecessary tags, duplicate styles, etc. from HTML documents before serving them.
initcwnd
The initcwnd (initial value of TCP’s congestion window) is set to 10 for
better utilization of available bandwidth over the network. Most other servers
use an initcwnd value of 3 by default. See this
article
for more details.