The Lightmap Snapshot contains the GI input data and all the intermediate files needed when creating the lightmaps for a scene. The snapshot 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. The snapshot is an Editor only construct so far, so you can’t access it in the player. When you change the scene, for instance by breaking a prefab connection on a lightmap static object, the snapshot data will get out of date and has to be rebuilt.
Currently, this file is a bit bloated as it contains data for multiple platforms - we will fix this. Also we are considering adding some compression for this data.
Concerning the rebuild time, this will vary. If your GI Cache is 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 first which requires the precompute and bake processes to run. These steps can take some time.