Saving is instant whatever the size. Loading is quick for everyday setups, from a handful of nodes to a couple of hundred. Very large trees take longer because Blender rebuilds nodes one at a time. Organizing complex work into node groups keeps things fast, and Node Vault rebuilds those groups faithfully.

The nodes still load, with that slot left empty and a warning telling you what was missing, so you can reconnect it. Image textures are also stored with their file path, so they are reloaded from disk automatically when the file still exists.

Yes. Send them the .json and .png pair for that setup and have them drop both into their own library folder. It appears in their gallery. There is no account or cloud service involved.

In a plain folder on your machine, which defaults to Documents/BlenderNodeVault and can be changed in the add-on preferences. Each setup is one .json file plus one .png thumbnail. Point the library folder at Dropbox, Google Drive or any synced folder and your library follows you between machines.

Yes, including groups nested inside other groups, to any depth. Their full interfaces come back too: sockets, panels, default values and min/max ranges.

All four node editors: Geometry Nodes, Shader, Compositor and Texture. The gallery automatically shows only the setups that belong to the editor you are currently working in.