@rkim--

Iframe page loading is asynchronous. So, use the listeners : { } to wait for the dataavailable event to fire, THEN interact with the frame document (see the example page included in the zip for other samples)


Code:
{
      xtype   : 'miframe',
      src       : 'yournestedExtpage.html',
   
      listeners: {
                dataavailable : {    //raised when the document DOM is ready (same-origin only)
                    element : 'iframeElement',  // MIFElement reference
                    scope    : 'iframeElement'',
                    fn : function(e, target){
                        var win = this.getWindow();
                        if( win){
                              if( win.Ext && win.MyApp ) {
                                      win.MyApp.iframefunction();
                              }
                        }
                    }
                }
        }
  }
1) Is above the recommended way of accessing functions in iframe document ?

Never add (expando) new methods/props on the document object. If you are truly loading ANOTHER Ext page into the frame, setup a namespace to keep your variable and functions:
//

yournestedExtpage.html
Code:
<script>
 Ext.onReady()
  {
    Ext.ns ( 'MyApp' );
    MyApp.iframefunction = function()
    {
       alert("Test");
    }
  }
</script>

2) Is there any performance issue of loading each page in ManagedIframe?

Yes, IFRAMEs are expensive to operate. Use them to isolate CSS/bad behavior from your parent page. There are good reasons to use them (adWare, media, PDFs, mashups/portals, legacy App migration, etc), but they do take their toll.

3) Is there any document that I can reference other than looking at code ?

My signature block below has links the documentation site and FAQ for the previous release. As this is still an Alpha release, there is not yet official docs for this version yet. There are several new feature but the MIF.Element methods are largely the same.

Study the zip-enclosed mif.js example closely. Much can be garnered from that.

Good hunting