if the light object doesn't start with sweethome3d_light_source, won't it then show in the SunFlow render?

Yes, only the 3D parts starting with sweethome3d_light are invisible when the object it belongs to isn't selected, and the photo renderer works on a copy of the home where everything is unselected.

Sorry for my misleading previous message, but great to learn that optimization on the rendering parameters brought you such a faster processing. Did you downscale the image you published here, or is it full scale?