Concurrent user expectations

[scottgmo]

We did a training session on our UAT envronment with 12 users recently and discovered severe degradation in performance.  Reports which ran under 10 seconds were taking over a minute to run.  After trouble shooting the issue some, we identified our Cognos environment as the bottleneck.  During load testing, the database was pretty much idle while Cognos CPU was maxed out.  Based on posts I've seen I'm inclined to think we are underpowered for 12 users.

Our UAT environment is 2cpu, dual core, with 4GB of memory on AIX.  One post I read on another forum suggests this setup is adequate with Cognos default settings and can support 8 concurrent users.  Our expectation was that we could support way over 12 (like 30 to 50).  Also, another post I read suggests you should have 1 CPU for every 2 concurrent users.  Our data models are strictly star schema and we are pretty much delivering the answer to Cognos so there is little local processing.

Does anyone have thoughts on this?  Are there any rules of thumb, formulas others have applied to scaling their environment.

Thanks in advance for any insight on this.