Everyone wants their website to be fast. For many years, we had synthetic tests: you spin up some automated browser, load your site, measure, done. It's not perfect, but it's a pretty good indicator for how fast your website is. You can simulate slow networks. You can run this automated browser on a different continent. It offers some flexibility.
But then someone convinced us all that it isn’t enough. You have to actually measure the peformance in the browsers of your users. RUM came into the world. From MDN:
Real User Monitoring or RUM measures the performance of a page from real users' machines. Generally, a third party script injects a script on each page to measure and report back on page load data for every request made. This technique monitors an application's actual user interactions. In real user monitoring, the browsers of real users report back performanc