That is 284mph. That is certainly faster than the terminal velocity of a level airplane, I suspect even if it was in an unpowered nose dive that would be too fast. 12 seconds is 5000 feet above the water level, and I understand it turned when it was at about 8000 feet.
Normal speed below 10,000 is 250 kts then slowing to 210 kts prior to extending flaps and gear and slowing to its final approach speed. This is really strange.