Hey Hara, There are lot of factors for your flex app performance: 1) Server response, 2) Backend response, 3) What kind of communication are you using in the middle tier? FDS or open AMF or some other 4) Container nesting & architecture 5) What is the app loading initially? By this I mean whether you are loading an image, etc..
You need to revisit your application architecture & pin point the issues. From the front end, yes, container nesting plays a very important role & also the type of controls you are using. Hope this helps. Regards