My understanding from reading this is the complete opposite. This library is explicitly not supporting the features that web servers need to be performant and handle production traffic, like streaming.
A web server using this could only start parsing when it receives the last byte, and could only start responding when it’s done serializing, all while holding non-lazy trees of JsonValue objects in memory.
I suspect the vast majority of services are not dealing with such massive JSON documents that doing serde on them becomes a material part of their latency breakdown.
I've never built or worked on a service where the JSON payloads were so large that (de)serialization accounted for a significant portion of the timing profile
I'm sure lots of them exist, but for your typical CRUD API, this has not been a phenomena I've run into.