Let's make sure we're aware of blocking code in the main thread that causes scroll not to be smooth. Especially during the first view seconds of page load it is critical that we have a smooth scroll for the content loaded thus far.
For RUM data, we can consider the Long Task API (T168293). This task, however, is about detecting this for page views from our WebPageTest runs. This would most likely involve some kind of analysis on the DevTools timeline collected from the browser.