Steps to reproduce:
1. Open User-supplied project
2. Open Scene ("Transform_10000) with 10000 objects
3. Run Test -> Measure Open/Close Current Scene and Inspect the time in Console Window
4. Open Scene ("Transform_50000) with 50000 objects
5. Run Test -> Measure Open/Close Current Scene and Inspect the time in Console Window

Expected: the time scales linearly
Actual: the time does not scale linearly

Reproduced in: 2017.4.33f1, 2018.4.12f1, 2019.2.10f1, 2019.3.0b8, 2020.1.0a9

Notes:
Reproduces on Asset Pipeline V2 and V1

Reproduces on Mixed, Force Binary, and Force Text Asset Serialization

* Loading prefab instances is more expensive than loading game objects.
* Saving prefab instances is significantly faster than saving game objects.
* The time Unity requires to load Gameobjects or prefab instances does not scale linearly with the number of objects.
* The time Unity requires to save Gameobjects does not scale linearly with the number of objects.