The demo code stopping after it runs for approximately 5 minutes is concerning. I am worried it might be overheating; could you try running the demo in full-step mode with the current limit set to 3000mA and see if that gives you different results?
As far as the behavior you observed when you adjusted the delay, it sounds like you are adjusting the delay that determines the step rate. If that is the case, please note that the maximum speed is going to be mainly limited by your particular motor and operating voltage, although other factors in your setup can impact it too, such as the current limit, acceleration/deceleration limiting, decay mode, and what kind of load is connected to your stepper motor. You can find some additional detail in this post by Ben, but ultimately, it does not sound like it is related to your problem.
Brandon