You may already know this, but the Iray rendering tracks the path of light as it leaves its source and travels around to light up the world. So anything that reflects light (mirrors, chrome, the window frame that was causing difficulties, etc) will tend to increase your render time. So will enclosing your space (i.e. having ceiling, floor, and all 4 walls). Reflections and enclosures basically keep the light going and within the scene. Matte finishes that basically absorb light or getting the light traveling where it has moved beyond every object in your scene allows the engine to stop tracking that particular beam.
Transparency and refraction also cause extra calculations along a light's path to increase render time, but don't add to the potential for looping light paths.