Looked at both. ffjson is a library that generates the marshaling/unmarshaling Go code for your structs, so it doesn't support my use case at all. megajson is abandonware ("This tool is unmaintained. Please use ffjson instead").

go-codec [1] is very good, and can do things like interning string keys to reduce allocation overhead, but it's not faster than encoding/json when working on plain maps.

[1] https://github.com/ugorji/go