This would have given me the indication right away that there is a strong possibility of a contention issue.

You said that you tuned the queries subsequently, how do you know this? Are you statistics on the underlying tables up to date? What happens when you try to run just one report (for one client) manually... do you see the same issue? I would here looking at this and other things like indexing before looking at the actual report.

Another thing you may consider is the need to have all of the reports rendered complete right away. What is keeping you from spacing these out by, say five minutes? If you start at midnight (Assuming all month data is in) and you do twenty reports every five minutes, then you could be done by 9am the next morning. I mean what is your SLA for delivery of these?