Hi Equipo Desarrollo,
We are aware of that problem. It is only reproducible in the Silverlight module of RadAsyncUpload and is due to some malfunction in the way Silverlight reports upload progress. We are working on fixing it, meanwhile you can try disabling Silverlight should you need fluent progress update. You can disable Silverlight via placing the following script before the RadAsyncUpload declaration:
Greetings,
Genady Sergeev
the Telerik team