Comment by eklitzke

This is true but the relative overhead of this is highly dependent on the protobuf structure in one's schema. For example, fixed integer fields don't need to be decoded (including repeated fixed ints), and the main idea of the "zero copy" here is avoiding copying string and bytes fields. If your protobufs are mostly varints then yes they all have to be decoded, if your protobufs contain a lot of string/bytes data then most of the decoded overhead could be memory copies for this data rather than varint decoding.

In some message schemas even though this isn't truly zero copy it may be close to it in terms of actual overhead and CPU time, in other schemas it doesn't help at all.