Crosstab export to Excel Issue

[Ayush12]

Hi,
I am seeing issue when I export my Crosstab to Cognos.
I have a big report, with 100 + metrics, but I need to show only the ones selected in the Prompt Screen. So I have used "Suppress Nulls and Zeroes" option, and based on the user selection in UI, I make my metrics as nulls in my query data items.
The slowness occurs randomly, sometimes some combo of metrics runs fast, other times it is very slow. The problem is not so much when the output is HTML,but as soon as its EXCEL , i start seeing problems.

Thanks for the Help
Ayush