Increasing Prefabs lowers FPS (frame rate)

I have a fence that I would like to add to my scene and it is composed of a single prefab (with 192 triangles and 1 material). When I add the fence to my scene (~54 duplicates of the single prefab), my frame rate decreases significantly. The numbers are as follows:

No Fence Draw Calls: 1934, 60 FPS, Batched: 32

Add Fence (55 duplicates of PF) Draw Calls: 1940, 39 FPS, Batched: 3238

Add Fence with Combine Children script on Fence Prefab Draw Calls: 2074, 54 FPS, Batched: 80

I'm a complete noob and have just started using combine children on meshes in my scene which I will probably try and replace with static batching and possibly occulsion culling (which I understand might perform better). From what I understand, I shouldn't use Combine Children on a PF that is going to be dynamically batched. But why the huge frame rate hit when I add multiple instances of the prefab to my scene????