Reaching a 100% score is a bit hard and most likely it's not enough to just use a plugin, additionally it is also directly related to server performance.
However, in most cases you can reach a score above 90, which is an optimal and perfectly reasonable result, remember that Google will mark sites as 'slow' if they have a score less than 50.
Having installed JSpeed with all optimizations enabled on an average server, several websites have been escalated from a score between 40 and 50 to a score greater than 90.
To reach a high score with a Joomla website on the Google PageSpeed test, you should take care of the following best practices, all obtainable using the JSpeed plugin:
- Minify stylesheets: it's important to reduce the size of CSS files as much as possible
- Minify HTML: the page source code can also be minified to reduce the total size and bandwidth usage
- Optimize images: avoid to use heavy images that increase a lot the total page size
- Use lazy-loading: images and iframes that are out of views can be loaded only when the user scrolls down the page to reveal them instead that on the page load
- Take advantage of HTTP/2: the second version of the HTTP protocol includes several enhancements such as the preloading of resources
- Use a CDN: you can setup a CDN service of your choice and make all assets loaded by Joomla to point to that external CDN instead that to your website
- Activate the 'System - Page Cache' plugin of Joomla, or use a third-party plugin such as JotCache to cache the whole output to the browser
These aspects can be addressed by the Joomla plugin JSpeed, as a result you could be able to reach a very high result in the Google PageSpeed test: