Android right approach : where JSON response should be parsed - in UI thread, or in another one?

It is prefered that, every task that takes long time, should be proccessed in another thread to avoid overloading MainThread:

AsyncTasks should ideally be used for short operations (a few seconds at the most.) If you need to keep threads running for long periods of time, it is highly recommended you use the various APIs provided by the java.util.concurrent package such as Executor, ThreadPoolExecutor and FutureTask.

So if you know that you have big data and it will take time, you will use new thread, but if the data are small and takes less time, why take the risk? Move that to the new thread too

If, as you say yourself, the JSON data could be huge, and it could take some time to process, I think you could (or should?) try to process it in an AsyncTask. By doing this your UI thread will not be frozen during the processing.

In most GUI designs (not just Android), there are several threads having different roles and responsibilities:

• The "main thread," running at "normal" dispatching priority, basically has nothing else to do but to respond promptly to the demands of the user-interface system. When "messages" arrive for its consumption, this thread immediately pre-empts the other threads so that the message can be processed quickly. Like any good manager ... ;-) ... they don't do the work themselves. They pass it off to other people.

• When asynchronous requests (JSON ... etc.) are involved, there's usually a small "pool" of threads who are responsible for sending those to the host, receiving the response, doing the encoding/decoding, and then either acting-on the response or passing it along. These threads spend nearly all their time waiting on the host. They operate at a slightly-inferior dispatching priority.

• Worker threads, operating at an even-more inferior priority, do any work that is computationally time-consuming. As much as possible, these threads don't do much I/O. They give-up their time slices quickly and readily to any other thread, but they usually consume their entire time slice when they can get one.