Previously I used XML soap for data exchange in my web services, But it taking so much time to display data.
In what case would you need to use XML or json for web service response?
Definitely JSON. Try this library - https://github.com/johnezang/JSONKit
Also, this is probably a duplicate of Is parsing JSON faster than parsing XML .
Definitely use JSON web service.. You'll find its significance over SOAP web service once you use it..