Making a prefab kill the framerate

I have notice an strange behaviour in Unity.
I have an enemy ship gameobject and the player ship gameobject in my scene (and also a simple mesh terrain without any other object)
Before making a prefab with the enemy ship, I get 800-900 fps, but if I make the prefab (of the enemy ship) the fps drops to 240-260.???
It is the only change I make and the difference of frames per seconds measured in the stats is incredibly big…
Any idea?

What exactly to do you?

By creating a prefab, do you mean that by adding the enemy ship without it being a prefab the FPS is 800? (drag drop on the scene the original object) Or is the scene without an object and you instantiate a prefab?

I mean if I instantiate an enemy ship in the scene (hierarchy) and then I break the prefab connection when I enter play mode the fps raises from 250 (running the prefab enemy ship) to 800 (running the enemy ship no-prefab), do you understand me?

The 800-900 is only the graphics FPS, and those value are pretty unrealistic.
What is the difference in drawcalls with and without the new model, and what happens if you add a few more?

They are practically the same. The problem are not the drawcalls…