You can only improve what you measure. A few free tools will run the diagnosis for you.
The go-to tools
- PageSpeed Insights (Google): scores your site and lists what to fix.
- GTmetrix: detailed analysis of the loading process.
- WebPageTest: advanced testing with a full request waterfall.
What to look at
Beyond the overall score, keep an eye on loading time, total page weight and the number of requests. The « Core Web Vitals » (LCP, CLS and so on) are the metrics Google tracks.
Method: measure BEFORE any optimisation, write the numbers down, then measure again after each change.