If you export content from Report Designer to Microsoft Excel and the report takes a long time to generate or the process runs out of memory, you might be generating a report with more than 60,000 records.
For better performance, if you have a report that generates an Excel worksheet with more than 60,000 records, you must export to the XLSX format instead of the XLS format. Starting with Report Designer v10.2, Microsoft Excel XLSX is the only supported worksheet format.