The mail merge functionality is in our plans. However I cannot state when exactly it will be available.
We are planning to support merging with repeating sections. Datasource would be any object that implements IList.
If you want to support more complicated scenarios, you could try generating your documents manually using our document model.
About the performance - we are working constantly on improving it and will be very happy if you provide us with more details on the tests you have made as well as on what machine/OS they were made.
You can also test the performance of our demo app and see if you experience the same issues.
the Telerik team
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