It seems to be based on CrUX data, which means FCP (first contentful paint) and FID (first input delay).
The pattern since in the graph above does bear a strong resemblance to the report rate we've been getting of the different Chrome Mobile versions being rolled out over the same period:
Now, looking at First Paint, which for us is pretty much the same as FCP on Chrome Mobile over that period, it does seem to get gradually worse over time:
Now, Safari doesn't support First Paint and Firefox Mobile gets too little traffic, which leaves us with Opera Mobile as the only "other mobile browser" comparison for this. Opera is also Chromium-based, but lagging behind in terms of version (the latest stable version seems to be based on Chromium 73). And there we see:
At this point this would suggest that the last 2 versions of Chrome are likely responsible for the regression. Which is no less serious, since Google takes page speed into account for ranking purposes (which explains them releasing this new tool).