Early Portal results
Original (StaticLightingScale .1, Production, IndirectLightingQuality 1). Took 30s to build.
Portal setup
With portals, IndirectLightingQuality still set to 1! Took 90s.
The portals don’t change the brightness, they just tell Lightmass where to look closely for lighting. In this case with all the skylight coming in small holes they make a big difference. In a more open scene they wouldn’t be any good because the openings to the sky are large.