I found exactly the same problem as you. I didn't try simulink, but using matlab, I am not able to process in real-time even with low sample-frequency. For some reason the command: output_content = stepImpl(s, input_content); takes too long time - no matter what length the buffers have. The time should only be a bit longer than: bufferlength*sampleTime, but it is much longer. That prevents real time operation. I guess it is a matlab problem since the ADI oscilloscope seems to work ok.
Did you find a solution.