Well ... The application zoom in before it receives data or it can re-draw the graphic symbols
What you can do to solve the problem:
1. Get a faster internet connection (client PC)
2. Get a good graphic card (client PC)
3. Get a faster processor (client PC)
4. If you have your own services, do 1, 3 to your server.
5. All the above
What you can not help:
1. Speed of ESRI's services or third part services
However, you also can use the zoomStart event of the map to make symbol invisible and make it visible again once zoomEnd