Why does accessing www.text blocks the coroutine using WWW class?

I have written the following co-routine that send a JSON post request to webservice that also return a JSON object which can be 30-40MB large containing some data. This JSON object contains a base64 string which can be decoded into binary data of any type. After decoding, I want to saved this binary data as a file on the local machine. I want to make the whole process as asynchronous as possible and so far the download progress from WWW.progress is working fine but I encounter a strange block when accessing WWW.text:

 IEnumerator WWWJSONPost(string url, string jsonString)
     {        
                 
         var encoding = new System.Text.ASCIIEncoding();
         byte[] formData = encoding.GetBytes(jsonString);
         
         //Create the headers for this POST
         Hashtable theHeaders = new Hashtable();
         print("Request Headers:");
         theHeaders.Add("Content-Type", "application/json");
         print("Content-Type,application/json");
         theHeaders.Add("Content-Length", formData.Length);
         print("Content-Type,"+formData.Length);
         theHeaders.Add("Accept", "application/json");
         print("Accept,application/json");
         
         string userpwd = user + ":" + password;
         string encodedUserpwd = Base64Codec.EncodeTo64(userpwd);
         theHeaders.Add("Authorization", "Basic " + encodedUserpwd);
         print("Authorization," + "Basic " + encodedUserpwd);
         //++++++    
         
         //The blocking WWW object that initiate request upon construction
         WWW theWWW = new WWW(url, formData , theHeaders);
         while(!theWWW.isDone)
         {
             //Report progress
             if(m_ProgressHandler != null)
             {
                 m_ProgressHandler(theWWW.progress);//Works
             }
             yield return null;
         }
         
         yield return theWWW;

 //When www.text is access, all following codes are blocked for a while (1-2 sec)
         print("Output...");        
         //Do something with received data
         print("WWW byte count: " + theWWW.bytes.Length);            
         if(theWWW.error != null)
         {
             print("Error: " + theWWW.error);
             yield break;
         }
         
         print("Deserializing...");
         byte[] WWWBytes = theWWW.bytes;//Very fast, does not block
         string JSONString = theWWW.text;//blocks for awhile for a www with byte count 37205271, when this line is commented, no blocking occurs
         yield return null;
     }

Initially, I thought that creating another coroutine to decode the www.text would work but it did not. Now I'm pretty much stuck here. Is there anyway to prevent the block or should I just allow it to happen?