Let's add another User Timing API mark (performance.mark) for when the base-modules request comes back. E.g. in its callback, or in an RLQ callback.
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Closing per T192623 and T160315. For real-users, this isn't meaningful on its own given meaningful metrics (firstPaint, mwLoadEnd, domComplete) can remain stable or improve, even if this middle-point moves forward or backwards.
It is still useful on timeline, though, but both WPT and dev tools show that already with network information, which is now integrated in timeline views.