The only way in your code is to decrease $fn only for the sphere you are
using for the minkowski operation.
sphere (r=1, $fn=10);
Increase it in very small increments - you are trading off processor time
for smoothness, as it were. Depending on the size of a 3D printed part, you
may not see the extra smoothness of a high $fn value.
For example, your code with the the above modification took about 20 seconds
on my PC, and your original code is still spinning.