Slow Stepper motor rotation

Hi,

I am using and Arduino uno plus the adafruit motor shield to control a stepper motor. I installed the ML HW support package for both. The motor is connected as specified and supplied with 6V. It moves similar to whats shown in the examples online. Although I cant increase the rotation speed by very much. RPM stays in the single digits altough the motor should be able to do much more. When I increase voltage the motor starts to run very rough. If I tell it to run at 200 rpm one can see that rotation is not continuous anymore and the motor looses steps.

I suspect some timing issues but cant find an error.

I am sticking the the sample code that comes with the package