Export of report result hangs in Web Portal

I want users to be able to run report, view report result and export it to CSV file in Web Portal.
I developed a report: now users can run it and view its result. They can also export the result. For 10000 rows it works fine; size of result CSV file is 1.5Mb; export takes 1.5minute.
But when the number of rows is large (e.g. 25000) export just hangs.

Are there any settings in Web Portal\IIS that can fix this problem?