I don't think there's any way to do that. Perhaps it's better to decrease the loading time anyway Are you creating items on the server and it takes 20 seconds to transfer them to the browser?

Ok, perhaps you could add the nodes in several stages so that the user can see something immediately, as long as you know the nodes' positions from the beginning. E.g. when all data is loaded, sort the records by how far nodes are from the origin of the diagram, and add the first several nodes to fill the visible part of the applet. Then you can run a JavaScript timer that adds the rest of the nodes in chunks of say 25 or 50. Since they are sorted, the next nodes will probably be already created if the user scrolls down.

To avoid using much JavaScript to Java interop, which is not very fast, make sure you are initializing items properties through their defaults in the Diagram, instead of calling setters on individual objects after creation. E.g. set Diagram.ShapeBrush once instead of calling node.setBrush for each node.

Also if you are running some layout algorithm, you might try using its faster variants. E.g. LayeredLayout is faster when EnforceLinkFlow is enabled, and OrthogonalLayout is much faster if you set MinimizeLinkBends to false.

I hope that helps,
Stoyan