A Lighting Data Asset stores precomputed lighting data for a Scene in the Unity Editor. Lighting Data Assets exist as separate files in your Project for workflow reasons; storing precomputed lighting data in a separate file means that changes to the precomputed lighting data do not result in changes to the Scene file. Lighting Data Assets are not intended for users to edit.
Unity stores precomputed lighting data in a Lighting Data Asset when you manually invoke a lighting precompute, either by using the Generate Lighting button in the Lighting window, or by using the Lightmapping.Bake or Lightmapping.BakeAsync APIs.
The Lighting Data Asset contains the GI data and all the supporting files needed when creating the lighting for a scene. The asset references the renderers, the realtime lightmaps, the baked lightmaps, light probes, reflection probes and some additional data that describes how they fit together. This also includes all the Enlighten data needed to update the realtime global illumination in the Player.
When you change the scene, for instance by breaking a prefab connection on a GameObject that is marked as Contribute GI, the asset data will get out of date and has to be rebuilt.
The intermediate files that are generated during the lighting build process, but is not needed for generating a Player build is not part of the asset, they are stored in the GI Cache instead.
The build time for the Lighting Data Asset can vary. If your GI Cache is fully populated i.e. you have done a bake on the machine before (with the scene in its current state) it will be fast. If you are pulling the scene to a machine with a blank cache or the cache data needed has been removed due to the cache size limit, the cache will have to be populated with the intermediate files first which requires the precompute and bake processes to run. These steps can take some time.