Start with google PageSpeed Insights, It is the most reliable tool because it follows google ranking rules directly, enter your URL and you get a performance score with a clear list of what needs fixing, the suggestions are practical like compressing images or removing unused code so you know exactly where to begin.
For deeper analysis use GTmetrix or WebPageTest, these tools show a waterfall chart that breaks down every element loading on your page, you can see which files take the longest and where the real bottlenecks are, they also let you test from different locations which matters if your audience is spread across multiple regions.
Focus on Core Web Vitals like Largest Contentful Paint for loading speed and Cumulative Layout Shift for visual stability, these are direct ranking factors, run tests regularly not just once because changes over time can impact performance, a fast site keeps users happy and Google rewards that.