Not seeing your results

[brandf]

I'm parsing a large json file and I want to keep my UI thread un-blocked. My json is about 25mb.

I'm getting the JSON from a XMLHttpRequest and I'm comparing 3 methods:

1. responseType: "text" + JSON.parse
2. responseType: "json"
3. responseType: "text" + JSON.asyncParse

I'm measuring UI thread responsiveness by measuring the time delta (performance.now) between requestAnimationFrames. Any frames >150ms get reported. This will catch UI thread blocking in native code (for example method #2).

Here are my results:

1. one 350-400ms frame per XHR + JSON.parse (mostly in JSON.parse)
2. one 350-400ms frame per XHR (in native code, where it does the json parsing due to responseType)
3. one 1000ms frame per XHR + JSON.asyncParse (due to marshaling cost)

As you can see, json.async blocked the UI thread 3 times as much as JSON.parse. This seems to contradict your results. Can you reproduce your results measuring the delta time between RAFs?

[brandf]

I also tried a variation of json.async which creates a single persistent Worker and reuses it. This had the same results as #3, so I didn't include it.

[brandf]

Oh, and I'm running on OSX (Yosemite) + Chrome (v39) on a new Macbook Pro