I've been having great luck with the new spreadsheet tool. I have a report that works correctly with anything less than 1000 rows or so. I am not sure if it is my code that is causing the issue, or if it is related to another post (http://forums.adobe.com/message/3318663#3318663).
Basically I am getting a list of accounts and then looping through those accounts, adding a monthly total, and then adding grand total for each account. If the account (or multiple accounts) has less than 1000 rows, it takes all of 1 second for it to spit out my report. If the account query has more than 2000 rows though, even with setting my timeout to 5 minutes, I will get nothing. I am using .xlsx format because the final report with have around 200k rows.