Generally, Report processing and rendering are extremely memory- and time-intensive operations. During the report generation, there are newly created resources. Each item takes memory and time to be processed and rendered in the report. The operations are very complex and include data processing (filtering, sorting, conditional formatting, etc.) and rendering (fonts measurements, layout calculations, paging and etc). For more information on what and when happens you may check the Report Life Cycle article.
A lot of factors will influence the report's performance: machine capabilities, number of report items, amount of data, report's complexity and rendering format. For more information on how the report could be improved, you may check Performance Considerations help article.
Consider also switching to PrintPreview mode - see Interactive vs. Print Layout. This display mode respects physical paging and will display the first physical page after it is ready, and the rest will be rendered in the background. The default Interactive mode renders the report with soft page breaks only, hence the first page may be huge, for example, if it is a table with thousands of rows.
Another way to improve the performance is to run 64-bit version of the Standalone designer. The reason is that 32-bit applications are limited to using up to ~1.8GB of memory - check Memory Limits for Windows and Windows Server Releases.
You may use Filtering through Report Parameters on a database level to reduce the amount of data. By selecting different values for the filtering parameters you will be able to render different parts of the big data.
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