> Also, hard to say without source code but there is a high chance even more efficient methods like Protobuf create a lot of Java objects and that kills cache locality

I don’t think this can be claimed that easily without more info, generational GCs work pretty much like an arena allocator, with very good cache locality (think of an ArrayList getting filled with objects that are continuously allocated in short order. The objects will be right next to each other, in memory). If the objects are short-lived, they can be similarly cheap to stack allocation (thread-local allocation buffers that just bumping pointers).