It's exciting working at SpeedCurve and pushing the envelope on performance monitoring to better measure the user's experience. We believe when it comes to web performance it's important to measure what the user sees and experiences when they interact with your site. A big part of our focus on metrics has been around rendering including comparing TTI to FMP, Hero Rendering, and critical blocking resources.
The main bottleneck when it comes to rendering is the browser main thread getting blocked. This is why we launched CPU charts for synthetic testing over a year ago. Back then it wasn't possible to gather CPU information using real user monitoring (RUM), but the Long Tasks API changes that. Starting today, you can track how CPU impacts your users with SpeedCurve's RUM product, LUX.
We also measure the CPU usage to different key events in the rendering of the page. SpeedCurve's focus is on the user experience and getting content in front of people as fast as possible, so we show you what the CPU is doing up till the page starts to render. This reflects CPU usage during the browser critical rendering path and can highlight various issues. If there's lots of CPU idle time then you're not delivering your resources efficiently. You want to get the CPU busy nice and early rendering the page, rather than sitting idle waiting for slow resources.
In the test below we see in the first pie chart that the CPU is spending a lot of time on layout up to the start render event, which is quite a different picture from the Fully Loaded CPU usage.