JSON-RPC is not about performance. A RPC protocol just makes more sense for full-duplex communication where both sending and receiving end can send messages to each other at any time. REST is inherently a request / response pattern and is not designed to build these kinds of "real time" applications. It's about your system architecture. Better to use the right tools for the job.
Can anyone shed some light on how big of a performance boost JSON-RPC provides when dealing with things like notifications?