Corpses are not actually objects in game, when you see a unit die a certain way, like a marine having an acidic death, its actor references a model to be played as its death animation, the model being a marine having an acidic death. If your asking to make these go slower, then simply change the animation speed of the model to a lower value.
If you want infested terrans cast by Infestors to last longer, change the Behavior applied to them in their Create - Unit Effect duration to a larger number, if you want them to be permanent, simply make it so the Behavior is not created on the spawned unit in the Create Unit effect.