Object Storage is not NECESSARY for smaller clusters.
Small clusters can use centralized metadata to manage storage very quickly.
Centralized metadata can support both POSIX and object semantics.
But when you scale up the metadata must be distributed, making efficient support of POSIX untenable.
That said, there is no inherent reason for "file servers" to be faster than "object servers".
The metadata operations required to put a new version of a 10 MB document are actually
less demanding with object storage than with file storage. The only advantage file APIs have
is their familiarity to developers and the maturity of the code base implementing POSIX APIs.
Neither the existing code or the APIs will scale to the degree that object APIs allow, however.