As part of a Reddit AMA in August 2018, Google’s John Mueller was asked a question on which tool to use and trust, with regards to monitoring the page speed of a site.
Specifically: “Google has said that only the slowest sites will be impacted by the coming Speed Update and referred us to 2 tools to check our speed, PageSpeed Insights and Lighthouse. These two tools return different scores. If a website is “average” in PageSpeed Insights, but “slow” in Lighthouse, which score will Google be using (especially for mobile)?”
John responded to this question with the following advice:
I would not focus on a single score for determining the speed of your site, but rather try to take the different measurements into account and determine where you need to start working (we also don’t just blindly take one number). There’s no trivial, single number that covers all aspects of speed. I find these scores & numbers useful to figure out where the bottleneck is, but you need to interpret them yourself. There are lots of ways to make awesome & fast sites, being aware of an issue is the most important first step though.