In recent benchmarks, Node's JSON implementation is almost twice as fast as messagepack. I think I'd rather have the more universally used format anyway.
It's not about speed, but space efficiency, msgpack is significantly smaller. That's useful for a mostly in memory database. As far as speed that's negligible.
Hello, if you care about size, the best way is to just use Redis Hashes, that internally store fields and values into a binary very compact representation.