I can see the export to excel feature in report definition has the setting to increase the limit for the number of records to export. Generally Number of rows * Number of columns should be less or around 10 lacks in an ideal case and nowhere it is mentioned but its practical value.
May be you can try to map your external table with a External class mapping wizard, then try to create the RD on this class. As 8lacks is a huge number you can try to set the number of records to export field with different limit value and check the time taken for the retrieval with lesser columns in the report.
May be this way you can try to export records more than 1.5lack.