Alain-
The performance metrics in the rails logs are not accurate at all. It is really hard to have a system reliably measure its own performance. The logs lie. Use httperf or ab to do any kind of req/sec testing.
Cheers- -- Ezra Zygmuntowicz-- Lead Rails Evangelist -- ez@engineyard.com -- Engine Yard, Serious Rails Hosting -- (866) 518-YARD (9273)