I’m not (at least it seems) experiencing 2x faster builds either. Current light build, 76% after nearly 30 hours (scene with 3 rooms, balcony and maybe 20 items furniture total) - corei7 4790k with 16gb ram + core i5 4460 with 8gb ram.
64 hemisphere samples, production mode, .2 static lighting level, 100 bounces, 10 indirect lighting quality. 2 lights in scene - direct and skylight, light portals used in windows.