Thank you for the sent project.
Setting the document and printing it further, requires the document to be paginated (measured and arranged). I've measured the importing and rendering of the document in RadRichTextBox and it takes about 135 seconds. I've logged requests in our backlog regarding performance improvements in such scenarios with large documents (around 1200 pages). You can follow the related items in our feedback portal in order to be notified about any related changes:
As a workaround for the performance issues, I could suggest you to print the documents separately instead of merging them into a single one and to notify the users (e.g. show a ProgressBar) about the progress.
Please get back to us again if you have further questions or any feedback.
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