The reporting engine in WPF renders the report with WPF primitives which are then displayed in the WPF report viewer. When there is a huge amount of those primitives on a single page (very large reports) you might experience delay when interacting with the WPF application.
In print preview the report is split based on the physical page size which is small enough so you don't see any performance issues when working with the same report.
If you use page breaks to split the report in reasonably sized parts I believe you will observe the same performance gain as seen with print preview.
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