I previously posted about my work to better understand web performance optimization, particularly with WordPress. In doing so I have used a number of online testing resources alongside my local implementation of Lighthouse and chrome developer tools.
Here are online performance testing sites I have used:
Web.dev and Google PageSpeed Insights give identical results for performance. Both use Lighthouse under the hood and based on my anecdotal testing experience, appear to use the same methodology for throttling to simulate mobile device performance. This is important, since you need consistency to evaluate any optimizations you try. When using Lighthouse on my local machine with the default throttling settings I get better, but self-consistent results.
GT Metrix also uses Lighthouse under the hood, but the presentation of the results differs radically from the above sites. You do not get the same performance numbers and mobile testing requires a subscription. Very useful features which are unique to this site are the ability to do a side by side comparison of different urls and a waterfall display.
Webpagetest is uses a different performance engine which may also be installed and used locally. I have just started using the online version and may have more to say about it later.