Decoding slower than JSON.parse()

I am loading a geojson file of full US Zipcodes over an ajax request, before I was using plain json, I now change it to use geobuf. So I replace the JSON.parse() by a geobuf.decode().

The document says it should be faster:

even faster than native JSON parse/stringify.

However now it is actualy 2 seconds slower.

var req = new XMLHttpRequest();
req.responseType = "arraybuffer";
req.addEventListener("load", function () {

	//var featureCollection = JSON.parse(this.responseText); // Took arround 4s
	var featureCollection = geobuf.decode(new Pbf(this.response)); // takes arround 6s
	//...
});
req.open("GET", url);
req.send();

Am I doing something wrong or is there any way to improve it?

Start with the XMLHttpRequest example in the issue, comparing JSON.parse(this.responseText) with geobuf.decode(new Pbf(this.response)) on the full US Zipcodes GeoJSON. Trace the decode entry point and benchmark both paths to determine whether the slowdown is reproducible and what work would be needed to improve or explain it.