A photo scenery has a mesh too !
The mesh is used for the elevation of the terrain, it's the shape of it: valley, moutains etc...
Then comes the choice for the textures:
- a simple Landclass, that will use the FS default ground textures and autogen
- a photo scenery, that will replace the FS default ground textures and autogen.

Which one is the most heavy on the perf ? Depends on the quality of the photoscenery:
- if the photo detail is not very precise, and there is no autogen, then it will be very FPS friendly.
- if the photo detail is very precise, and with a nice autogen on it, I would say it would consume more CPU power and RAM to be stored than the default FS textures.

This is just my guess. Please any expert correct me if I'm wrong.