Steps to reproduce:
1. Open attached “LightingPerformance.zip” Unity project
2. Open “Main” scene
3. Enter Play mode
4. Enable Stats in Game view
-notice the current amount of Verts and Tris with "meshSeparated" object enabled
5. Disable “meshSeparated” and enable “meshJoined” objects in Hierarchy
-notice the current amount of Verts and Tris with "meshJoined" object enabled

Expected results:
Number of Tris and Verts stays identical or similar
Actual results:
Number of Tris and Verts increases significantly

Reproduced in: 2018.1.0b1, 2017.3.0p1, 2017.2.1p1, 2017.1.3f1, 2017.1.0a1
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By design:
Unity culls per object, not per triangle. Therefore, separate objects can be culled from individual shadowmaps if they're not in the shadowmap's frustum which causes a reduced triangle count.