Why can't I toggle GPIO at high frequencies

I'm trying to toggle a set of GPIO pins on a F28335 DSP using Simulink (2012b). I notice that when I set the sampling time of the model to be high enough, the toggling frequency decreases.

If I set the step time of the model at 5 us, it will toggle every 5 us. But when I set the step time to be 1 us (5x increase in frequency), the toggle will only happen every 3 us. If I set the step time to be 0.1 us, there is no toggling happening whatsoever.

Is there some overhead or inefficiencies in Simulink-generated code that is keeping me from toggling these pins at high frequencies? The hardware should have no problem accommodating this since the DSP is on a 150 MHz clock. Any advice would be appreciated.

-HDL