In a previous project we used fixedint32/64 instead of varint values in the schema for messages where performance was critical.

That left only varint used for tags. For encoding we already know the size at compile time. We also don’t have to decode them since we can match on the raw bytes (again, known at compile time).

A final optimization is to make sure the message has a fixed size and then just mutate the bytes of a template directly before shipping it off. Hard to beat.

In a newer project we just use SBE, but it lacks some of the ergonomics of protobuf.