Performance in debug mode (F5) vs. no debugging (Ctrl+F5)

In your situation it means, that your application really stresses server for only 35%... other 65% that are stressed in Debug mode are stressed not with your app but with debug shell and debugger itself. If you're creating some kind of server tester for extreme situations - I recommend you to add to your app some module of diagnostic of CPU usage. So, that your app works till CPU usage got 100%, and then, may be, gives a report about at which condition CPU becomes 100% busy.... If you are using some predefined settings (number of threads, etc.) - then it means that your server is much more stronger that modelled situation, and you probably need to increase resource usage of your threads...