- The ScrollRect component in Unity's UI performs very slowly when its 'Content' contains children objects containing LayoutGroup sub-children objects.
- The drop in performance is noticed when the ScrollRect's 'Content' is being dragged up and down.

To reproduce:
1. Open 'scroll-rect-perf' in Unity (version tested was in 5.3.6p1)
2. Open the Profiler window in Unity, enable 'Deep Profile'
3. Open the scene 'ScrollWith1000Items'
4. Press the Play button in Unity to run the scene, NOTE: it's likely this scene will take a considerable time to start up
5. Scroll the container up and down by clicking with the left-mouse-button on any of the children objects and dragging the mouse upward or downward with the left-mouse-button still held down
6. Notice the performance in the Profiler showing much slower behaviour than the previous test. On average hardware, the performance should have degraded to the point of the end-user being unable to comfortably scroll up and down to elements of their choice.

Reproducible: 5.5.0b7, 5.4.1p4, 5.3.6p7, 5.2.5f1