Thank you for the quick reply.
Is there anything we can do to help speed up the process ?
Is there any workaround regarding this issue ?
@SimonSchwendele As I can see the issue is already escalated in the priority support.
As a workaround you can try using Merger class and MergeFormatMode.KeepSourceLayout to merge the documents.
Document docStart = new Document(@"C:\Temp\DocStart.docx");
Document docContent = new Document(@"C:\Temp\DocContent.docx");
Document result = Merger.Merge(new Document[] { docStart, docContent }, MergeFormatMode.KeepSourceLayout);
result.Save(@"C:\Temp\out.docx");