Depends on the way you handle JSON. You don't need to deserialize the entire received JSON string and then walk the node tree, you can walk the string as is. With this, you can process 500MB/s or more of JSON on a single core.

On the other side, you can produce JSON string directly, saving the need to produce some intermediate representation in memory.

You should be able to saturate 10gbit line on a multicore server easilly with this approach.