One of the ways of increasing visitors on a website is to lessen the amount of time that it takes a page to load. This also increases engagement as people are more likely to visit other pages on your website if they load quickly making it more likely they will find what they want.
The basic measure of this has generally been Google's PageSpeed Insights but for a lot of websites it isn't very good anymore. As an example of this, Mason Bee ranks as 100 on PageSpeed Insights but that is really all the information they give.
It isn't much, and the larger problem is that Mason Bee has been up for months and PageSpeed Insights has only just got got enough data from Lighthouse to make that appraisal. If you have a new site it doesn't tell you much at all.
Which brings us to GTmetrix. GTmetrix also analyze page speeds and then they give you an analysis of how well your site does and how to improve it.
Instantaneously it gives me more information about my page speed and what is slowing it down than Google does. As you can see I get a 98% PageSpeed score and a 92% YSlow score, my home page takes 2.4 seconds to load, is 695KB and takes 23 requests.
You will also notice there are tabs to give different views with more information. The YSlow tab shows me how to improve my timings. In my case it recommends the use of a CDN. Not actually something I need at the moment but possibly a good idea if my site traffic increases.
Another handy page is the Waterfall graph where you get to see just what the timings were as my server was contacted and the page was requested. There you can see that it took almost 0.9 sec to respond to the request meaning that almost a full second of my 2.4 sec page load time is spend waiting for the server. After that, most of the content served is pictures which is why a CDN is being recommended. Serving static content faster is what they do.
All in all I think that GTmetrix does a much better job of explaining what is going on with your website and it's loading times. It may not be owned by Google but to me it is clearly the better product. It is also free.