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I don't believe any formal stress testing has been done with xoops. But I would expect a number of variables to alter the conclusions
1. Raw Processing Power
2. RAM available
3. Network Throughput
4. Differences in performance in versions of Apache, PHP, MySQL
5. Architecture differences (apache / MySQL on same machine, apache / MySQL on seperate machines).
6. Modules utilized
7. External server load (additional applications / websites running on the server)
https://xoops.org currently has over 15000 subscribed users and the MySQL database currently handles over 60 requests per second. However it also is on its own dedicated dual processor server, with very little additional load.
If you are interested in running a stress test of XOOPS in your installed environment, I would recommend using
Microsoft's Web Application Stress Tool, which can be used to simulate multiple users accessing your site simultaneously.