Googles’ PageSpeed is a free Open Source tool to speed up sites at the server level.
It improves page latency and data usage via altering page elements to incorporate website performance best practices. Optimizations are done through custom filters that are called when the server sends website elements such as pointing to optimized images.
It has standard speed enhancements like ‘minifying’ .js and .css files, to more advanced features like truncating irrelevant meta-data from images and files, dynamically resizing them to certain dimensions, or even re-encoding them to be presented in the best format for efficiency.
They have a list of rules and best practices here: https://developers.google.com/speed/docs/insights/rules
The most convenient and useful tool is you can evaluate your site(s) here: https://developers.google.com/speed/pagespeed/insights/
The insights tool is amazing, and simple to use; enter your sites url and it will run. Once it’s finished its analysis, it will give your site two reports, one for mobile and one for desktop.
It will sort issues by severity, so you can see what is hurting your site the most right away. It then goes the extra step to show you exactly what’s causing the problems and offers suggestions for how to fix them.
They also offer PageSpeed as a server module here: https://developers.google.com/speed/pagespeed/module
Developers can use the module and configure every miniscule detail trying to eek the most performance out of a site.
Finally, here is a link in which you can compare your sites current speed to the optimization offered from PageSpeed: http://www.webpagetest.org/compare
Check it out!