There is no blame in this case, regarding the time duration of it.
As far as foot dragging, its probally the last thing any LEO wants to see or happen. However we are hamstrung with the Justice System and the previously mentioned factors. How you can say that the ordinary Military Policemen/Women or Investigators have contributed to them, only illustrates your lack of knowledge and understanding in the matter.
But in a case of Apples vs Oranges, Recruitment & Enlistment can take up to a year or more, And a year teaching you your left foot from your right, then God knows how long in PAT.
Four months on a Investigation resulting in the Death of Friendlies or Foreign Nationals seems like a wiz bang fast forward.