How to reproduce:
1. Open the attached "BugRepro.zip"
2. Open up the CPU Profiler and enable Deep Profiling
3. Start Editor Profiling
4. Enter the Play Mode
5. Turn off Editor Profiling and inspect the Performance spike in the Profiler Spike

Expected Behavior: The Play Mode loads fast and List'1.add() under InternalStaticBatchingUtility.Combine() has only 3200 Calls (400*8)
Actual Behavior: The Play Mode loads slow and List'1.add has 14400 Calls ((1+2+3+4+5+6+7+8)*400) even though only 3200 Objects are present

Reproducible with: 2017.4.30f1, 2018.3.12f1, 2019.1.0b10, 2019.2.0a11

Note: InternalStaticBatchingUtility.Combine() is called 8 times. If you put all the GameObjects into one scene - the issue does not reproduce.