Currently, there is no standardized way to measure Core Web Vitals and other web performance metrics inside single page applications (SPA) because they rely on soft navigations to respond to user actions, which are not as obvious to detect as hard navigations.
As of now, web performance monitoring tools can’t easily report web performance metrics for soft navigations. Instead, most metrics focus on the initial page load.
The main reason for this measurement gap is that we don’t yet have an agreed definition of what user actions qualify as soft navigations, which would allow developers of web performance tools and browser vendors to coherently detect and measure Web Vitals in single page applications.
However, there’s already some progress in the development of soft navigation reporting.
The Chrome developer team have started work on defining the heuristics and creating the APIs for reporting web performance metrics for soft navigations. The development is currently at an experimental stage, and the proposals are still not set in stone.