I played with the following 2 settings per your instruction. (BTW my mac mini is of late 2012 variety.)
Parallelize Sigma-Delta Modulator
This setting did increase the Processing Speed but it was minimal.
Enable Native DSD Processing
This setting showed whopping 5-fold improvement. The first picture below shows the Signal Path with this setting turned off. The second on.
No stuttering even @ DSD256 to 176 conversion (2.5x).
I do not discern sound difference between different combinations of the two switches. So that’s a good news. It is a cool finding and I thank you all for very timely and exact help. Muchas Gracias!