I agree that without sub-millisecond resolution, you cannot get the precise framerate based on the timestamps of two separate frames. You can still calculate the framerate by counting the number of frames per second.
I think this will be a good topic for discussion for tomorrow's conference call. If we do decide to allow for microsecond resolution, we should do so in the constraints of respecting the Unix epoch, so developers can easily compare timestamps with Date.now(), and be inclined to not change Navigation Timing unless it is for a very compelling reason.
I will add this topic to the agenda.
Jatinder