Costs of Static Lights on Render Thread

A Japanese developer's making a scene that has over 4000 static point lights.
They thought the lights don't affect rendering in runtime.
But they realized the RenderThread's costs become bigger.

I made a sample project that has 3000 static point lights to investigate this problem.
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And I realized that, as they said, InitDynamicShadows() in RenderThread checks all static lights, to know whether if there is any static light dynamically creates its shadow to preview.
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The developer said that it costs over 20ms on console on the scene they mentioned. Furthermore, the static lights wastefully increase memory usage and loading time. To avoid this problem, They separate the all static lights to another sub level.
(They understand the lights cannot be captured for reflection.)

Also reported in this thread:
https://udn.unrealengine.com/questions/300259/optimize-rendering-thread.html

1. Put a static point lights into any scene
2. Bake the light.
3. Play in the editor
4. send commands "stat startfile" & "stat stopfile" to profile it.
5. open the log with SessionFrontend's profiler.
6. You can see the static point light in InitDynamicShaodws() on RenderThread.