I just ran into the same issue with a different scene.
This time the poly count is lower (~ 10 million polys) so the IR slowdown is not as obvious, but we are still talking about an almost 50% performance loss as opposed to the "normal" render. 3,6 vs 6,6 Rays/s total.
I guess Frood already identified the problem and I hope there will be a solution for this.
