It slowed because the engine went "Tight". Its usually a tell tale sign that its gunna go boom! But at least now your driver has felt what a engine that goes tight feels like and the next time he will remember it and shut er down before the boom ! Sorry it happened that sucks only 5 passes. But I am willing to bed the valve guides were tightening up with heat.
We had this happen a few times when Ferrera had some faulty guides, I would take the heads apart and look for gauling on the stems, and inside the guides.