To reproduce:
1) Open attached project and the only scene inside of it
2) Play it
3) Open Profiler and select UI graph
4) Notice that Render and Layout times are roughly same and very low (so much that even an FPS counter is not shown, which caps at 10k)
5) Select Canvas -> ObjectWithRectMask in Hierarchy
6) Enable RectMask2D Component
7) Check the UI graph

Expected: Layout part of it doesn't grow too extreme and you save time as render calls are heavily reduced
Actual: Layout part increases very sharply (at least 10x)

Reproduced in 2017.1.0b1, 2017.1.0p1, 2017.3.0a2

There is no UI profiler graph in 5.6