There are large CPU spikes related to UI. While trying to track down the symptoms, User has found out that Canvas.BuildBatch encompasses every active canvas in the scene and subsequently slows SendWillRenderCanvases.
The problem is that all canvases are being rebuilt rather than only ones in the relevant hierarchy.

The event which causes this appears to be the addition of a GameObject to a ScrollRect's viewport. It's expected in that case that only canvas ancestors of that node are rebuilt, not all canvas in the scene. The codebase does not call Canvas.ForceUpdateCanvases().

Steps to reproduce:
1. Open User's attached project ("Test-ScrollandAnotherCanvas.zip")
2. Open Profiler
3. Enter Play mode
4. Wait for objects to stop appearing in scroll list
5. Pause Play mode
6. Look for Canvas.BuildBatch in profiler
7. Check Show Related Object

Result: Check that Overlay is being re-build

Reproduced with: 2017.2.0b3, 2017.1.0f3, 5.6.2p3.

Note: Sometimes it is easy to reproduce when the list is already filled, but sometimes not. So to reproduce it easily, move the scroll list when the items are being created.