Caching json strings in expensive objects

We are trying to optimize gson resource usage in our servers.
Most of our requests CPU's are taking 200ms just doing gson serialization.

We've done a great progress by using the following method:
http://stackoverflow.com/a/30811549/410293

Although, we can't figure out how to make this work when the object is nested into a JsonWritterTree. I was hoping you had some insight on how to possibly nest a string into the JsonWritterTree.write without AssertError.