Google PageSpeed Insights
With PageSpeed Insights you can identify ways to make your site faster and more mobile-friendly.
And this is also key to get a pole position in Google Search. The tool analyzes the delivered content of your website and makes suggestions to improve it.
More information about the extension: https://www.plesk.com/blog/introducing-google-pagespeed-insights-plesk-extension/
Detailed explanation of the PageSpeed Insights performance rules: https://www.plesk.com/blog/google-pagespeed-insights-optimize-your-site/