I just tried the latest daily (1.6 DailyBuild Mar 19 2017) and played around with the render regions on a really simple scene. I noticed that if I start an IR or resize an active IR, the scene noise clears up really fast, but if I add a render region or turn off all render render regions, the scene renders really slow.
Steps to reproduce:
Start IR
Add a render region
Resize the IR window (to clear the non-rendering area)
Move the render region or add another render region
Notice how much slower the new render areas are
Resize window again, boom: renders fast again
Actually, I just noticed that the whole IR window/moving regions is really sluggish and laggy, not just the render itself.