The JSON looks like this:

    {"coordinates": [
       {"x": 0.65, "y": 0.23, "z": 0.91, "name": "fwgzd", "opts": {'1': [1, true]},
       {"x": 0.45, "y": 0.78, "z": 0.22, "name": "alfsj", "opts": {'1': [1, true]},
       ...
    ],
     "info": "some info"}

The benchmark code [1] (very readable) is reading an array of structs containing x,y,z from 'coordinates'.

[1] https://github.com/kostya/benchmarks/blob/master/json/test_f...

I haven't read the code but the algorithm would look at this roughly like:

   See `{` process? yes
   See `"coordinates"` process? yes
   See `[` process? yes
   See `{` process? yes
   See `"x"` process? yes => new Coord, Coord.x = 0.65
   See `"y"` process? yes => Coord.y = 0.23
   See `"z"` process? yes => Coord.z = 0.91
   See `"name"` process? no, next token is `"`, skip to `"`
   See `"opts"` process? no, next token is `{`, skip to `}`

The tradeoff is that he's completely ignoring the contents of `name` or `opts` or `info` and those values could potentially be invalid JSON but this processor doesn't care.

The code is also picking up efficiencies from being a static C-like language. The "new Coord" isn't actually doing anything, the alloc happened for the array as a whole so the assignments just write a known size value to a known offset from the start of the array. He's also using SIMD instructions to process multiple bytes at a time and some other tricks but the skipping is the main difference.

I think it's also interesting that the Rust code implemented value skipping in the benchmark file itself. The relative slowness there is likely that the library used (serde_json) is the JSON plugin for a generic serialization/deserialization lib and that Rust doesn't have a way to do SIMD yet.