Any child being re-parented fires off a message to its CanvasGroup, the CanvasGroup then broadcasts a message to all of its children, this message causes each child to work its way up the hierarchy, calling GetComponent at each level, trying to figure out how it should handle its canvas group registration which results in slow performance.

Steps to reproduce:
1. Open attached project
2. Open "CanvasGroupMakesSetSiblingIndexReallySlow" scene
3. Select GameObject->Canvas in hierarchy
4. Add component "Canvas Group"
5. Start play mode
-Performance drops from 90fps to 40fps

Reproduced on: 5.5.0b5, 5.4.1f1
Not reproduced on: 5.3.6p5