var updateInterval = 0.5;

private var accum = 0.0; // FPS accumulated over the interval
private var frames = 0; // Frames drawn over the interval
private var timeleft : float; // Left time for current interval

function Start()
{
     if( !guiText )
     {
         print ("FramesPerSecond needs a GUIText component!");
         enabled = false;
         return;
     }
     timeleft = updateInterval;   
}

function Update()
{
     timeleft -= Time.deltaTime;
     accum += Time.timeScale/Time.deltaTime;
     ++frames;
      
     // Interval ended - update GUI text and start new interval
     if( timeleft <= 0.0 )
     {
         // display two fractional digits (f2 format)
         guiText.text = "" + (accum/frames).ToString("f2");
         timeleft = updateInterval;
         accum = 0.0;
         frames = 0;
     }
}