When the workbook is exported to Excel, it is currently not possible to display the progress for example, in percentage of exported data. 

You are returning a "jquery promise", you can call the ".notify" function while the process is running and tell us that you are at step X of Y, when running these settimeout.

https://dojo.telerik.com/mHMUMjpg/6  

So, we can display a nice progressbar, instead of just a "loading animation" that doesn't tell if the process is really running. In some cases, it can take between 30 and 60 seconds, so the user think that the app is doing nothing!