Nimble snapshots are mostly read-only, so mounting a read-only snapshot for Oracle DBs won't really give you the functionality you're looking for. Therefore it's best to clone the volume from a Nimble snapshot, as it becomes a full writable volume, but still does not copy any of the back end data associated with it. Essentially, it's doing what you want it to do for your dev DB instance, yet it's not invalidating, modifying or corrupting your original snapshot.
There's no performance differential between snapshots or clones, and neither have an impact performance wise on the parent volume.
Hope this helps answer your question?