Could you please provide guidelines or code snippet to use QNetworkAccessManager using multi threaded env.
No, I can't, sorry.
Not that I don't want to, but I don't know of any way you can control the number of threads QNetworkAccessManager uses internally (I think it's hardcoded). If I were to do multithreading with NAM I'd do it like you - thread the reply's processing. Unfortunately this bears no weight on the HTTP response time.
On a related note, I would always prefer to use TCP/IP directly, as it provides much more fine-grained control of what's happening (including threading). The downside of that approach however, is that it'd require implementing the HTTP protocol by hand.
Perhaps, as @p3c0 suggested, you could try to ask the question on the mailing list were you might get responses from the actual developers of the module.