What is the ruling on the one who says, concerning the swift Tarawih prayer: "This is not Tarawih; this is a game, and it is exhausting: standing and sitting, and bowing, standing and sitting, and bowing"? And is this statement considered disbelief, even without the intention of mocking the ritual of prayer?

As long as you did not say that out of mockery of the prayer itself, but rather out of disapproval of the imam's speed, then it is not considered disbelief. A Muslim should guard his tongue, especially when speaking about the rituals of Allah Almighty. Prayer becomes invalid due to speed if it violates the pillar of tranquility.