I lead engineering for some of Google's developer tooling like Lighthouse. You are correct that we currently have a gap in our ability to measure subframes in the lab locally on your machine (doing so cross-origin reliably is complex), but we're looking at how we can improve this. The important bit I want to call out here is that "framehole" is a measurement gap in LH: you can currently game the score but you're only fooling yourself in the process — there is no benefit to the user experience.
Speaking of real world user experience, accounting for iframes has a number of complicated privacy and security challenges at the resolution of an individual page load. However, the good news is that the Chrome User Experience Report* aggregates anonymized data from many opted-in users and, as a result, is able to account for iframes in its measurement. Which is to say, the field data we surface in our tools (powered by CrUX) does not have this problem. We’re working to get our lab tools to align with this behavior as well.
Speaking of real world user experience, accounting for iframes has a number of complicated privacy and security challenges at the resolution of an individual page load. However, the good news is that the Chrome User Experience Report* aggregates anonymized data from many opted-in users and, as a result, is able to account for iframes in its measurement. Which is to say, the field data we surface in our tools (powered by CrUX) does not have this problem. We’re working to get our lab tools to align with this behavior as well.
* https://developers.google.com/web/tools/chrome-user-experien...