Is there a better way to tilt terrain at runtime?

I'm using this code to tilt my terrain at runtime. It works pretty well except it's very slow. It has to iterate through each height point so it ends up taking 2-3 seconds. I'd love to find a way to shave even a little bit of time off.

 function OnGUI ()
 {
     if (GUI.Button(Rect(200,10,50,30), "tilt"))
     {
         SlopeAngle (0.1);
     }
 }

 function SlopeAngle (change : float)
 {
     var tData = terrain.GetHeights(0, 0, terrain.heightmapWidth, terrain.heightmapHeight);

     // iterate through height points
     for (i = 0; i < terrain.heightmapHeight; i++)
     {
         for (j = 0; j < terrain.heightmapWidth; j++)
         {
             // lowers terrain points at end, gradually decreasing to 0 as it reaches end
             tData[i, j] -= ((change / terrain.heightmapWidth) * j);
         }
     }
     terrain.SetHeights(0, 0, tData);
 }