Make your emissive relatively dim and place a real lightsource in front of or inside your emissive surface to cast the actual light. Currently you can expect noisy results by trying to light a scene primarily with emissives. At the moment there’s no solution other than to try to avoid situations like this.
It might be that this will be improved on release, supposedly the Matrix demo’s night scene was lit entirely with emissive and seems to do so without issue so perhaps there are improvements to this coming.