This is my test using Realistic Rendering demo from Epic. Using default settings in BaseLightmass.ini (no tweaking at all), World Settings Lightmass, and Matinee settings. The light building time was 12 minutes, without Lightmass portal.
The very same project took 25 mins to build the lighting in UE4.9.2, using default settings as well for .ini file and Lightmass setting. The light building quality was set to Production.
The only thing that I had to change was the SM_Background’s material, from Translucent to Opaque, otherwise all the buildings outside will be invisible. I think UE 4.11 has brighter lighting than all of the previous versions of UE4.
This video was recorded using Matinee at 1280x720 60fps. If you watch it in HD, the video will playback at 60fps.
UE 4.11 preview 1 still has unstable video (AVI) recording though (crashed several times), so I had to record it as a series of PNG files and then creating the video using Vegas Pro.