I found that it was too slow to scale the model on my mobile phone
What do you mean by “scale the model on my mobile phone”? I assume you’re talking about camera pinch gesture to zoom in and out? If so, take a look at ArcRotateCamera.pinchPrecision.
It might also be because your model is huge and the camera is far away to capture it all in frame? In that case, you could scale the model down and move the camera closer, then pinch to zoom should be faster.