The fix for bug 1283302 results in Gecko firing the before-first-paint notification earlier than before. The before-first-paint notification initializes the APZC painting code. By design, APZC will paint an area beyond the viewport bounds, which may account for some of the startup hit here. However, Firefox would incur this startup cost anyway (after 250ms today) so it seems wrong for Talos to be testing page performance without the first-paint step. Filing this bug to track that down.
added more alerts to this, we see this in Android as well, overall, I suspect whatever fixes we do for desktop will have the same effect on Android.
Is it still P1?