Please excuse me for the delayed response.
We have tested various scenarios similar to the one you describe - a flat tree with hundreds of nodes, a single node with hundreds of child nodes and different in-between versions. In all cases RadTreeView performs very well, without any noticeable delay when checking a single node. Could it be that there is something specific to your that we are missing in our tests? Are you using templates in your tests? It would be great if you can open a support ticket and send us the project you are testing so we can examine it locally.
As for the second issue, if your project has <compilation debug="true
"> set in the web.config file, the ASP.NET AJAX framework will use debug versions of its scripts which are considerably slower that the production versions. These scripts can cause RadTreeView to perform significantly slower and it's possible that the Stop Script message is shown by Internet Explorer in this case. If you set <compilation debug="false
"> in your web.config file, you should not see the message displayed for this number of nodes.
the Telerik team