Concerning 'slammed into concrete': I do not think that is correct. The blow seems too forceful for mere slamming.

Concerning: 'slow' or 'fast': I would agree that there is insufficient evidence to be certain, however, given the brutality and cowardice as well as considering the 'fun' involving the note, I feel it is proper to conclude that the most probable choice is 'slow'. Real slow!

As to 'imagining' what was done: that precisely is what he wants to happen. In the wee hours of the morning, even to this day, even when it is involuntary, he wants the Ramseys to imagine what was done.