Luminous

Use luminous geometry sparingly for lighting a scene - using lights is far more efficient. If the luminous object you want to use to light the scene is roughly spherical, add a Spherical Light for the effect instead. If it's more of a box shape, use an Area Light shaped to the rough dimensions. Only use a Primitive Light when you cannot roughly replicate the shape of your luminous object

Lights are directly sampled, and Importance sampled, by a scene's GI. Luminous geometry is not. The scene knows exactly where the lights are and samples them, luminous objects are only sampled when hit by GI calculations.