There are many free Tools that you can use to optimize your website. Here I would like to introduce you to some of them. Each of the tools shows in detail what exactly the problems are to improve them.

Optimize the page speed and performance

PageSpeed Insights
Lighthouse | Tools for Web Developers | Google Developers
Learn how to set up Lighthouse to audit your web apps.
GTmetrix | Website Speed and Performance Optimization
GTmetrix is a free tool that analyzes your page’s speed performance. Using PageSpeed and YSlow, GTmetrix generates scores for your pages and offers actionable recommendations on how to fix them.
Web Page Analyzer - free website optimization tool website speed test check website performance report from web site optimization
Free web page analysis service tests page size, composition, and download speed. Recommends speed improvements based on best practices for usability, HCI, and website optimization for maximum website performance.

Optimize SEO / Share

Sharing Debugger - Facebook for Developers
Google Search Console
Use Search Console to monitor Google Search results data for your properties.
Powerful SEO Analysis Online for your website

Certificates / SSL

SSL Server Test (Powered by Qualys SSL Labs)
A comprehensive free SSL test for your public web servers.