This is very useful, thanks! Go's slow JSON parser is annoying.

My only criticism is that returning 4 values from a function is not very ergonomic, especially since most of the return values aren't needed in many cases.

It would have been better and more idiomatic to return a struct. Alternatively, let it take a pointer to a struct — this would let you avoid memory allocation in loops (though in theory Go should be able to optimize a struct return, too; anyone know?).