macOS: nice/renice not working — no actual effect on cputime?

Question

Although I can see a "nice" value ("NI") via ps, it seems to have no effect on the actual cpu-time used by a process:

  PID  %CPU PRI NI COMM
57081  77.6  12 10 cpu-chew
57080  77.1  12  0 cpu-chew
57085  76.9  12 15 cpu-chew
57082  76.9  12 13 cpu-chew
57083  76.7  12  0 cpu-chew
57031   0.0  31  0 -tcsh

Does anybody have any idea how I can actually get one process to receive more CPU than another?

== Further info, fwiw:

MacOS 10.12.6

I get the same behavior if I try (re)nice'ing with a negative value, and also if I go up to more than more than 32 processes (just to make sure I'm maxing out "hyperthreads" or anything).

cpu-chew is a short C program that just does square-rooting and incrementing (see below).

Running 'ps' multiple times gives the processes in a slightly different order, and with a varying PRI value (from 1-ish to 14-ish), but all cpu-chew copies will have roughly the same PRIority. I gather that PRI incorporates some sort of age mechanism to account for time-since-last-switched-in.)

Here's the full transcript of me starting the jobs via different combinations of nice and renice:

tropic: cpu-chew &
tropic: /usr/bin/nice cpu-chew &
tropic: /usr/bin/nice -n 13 cpu-chew &
tropic: cpu-chew &
tropic: cpu-chew &
tropic: ps -rc -o pid,pcpu,pri,nice,comm

  PID  %CPU PRI NI COMM
57085  78.2   1  0 cpu-chew
57083  78.0   2  0 cpu-chew
57081  77.4   3 10 cpu-chew
57080  77.2   2  0 cpu-chew
57082  76.7   2 13 cpu-chew
57031   0.0  31  0 -tcsh

tropic: /usr/bin/renice -n 15 57085
tropic: ps -rc -o pid,pcpu,pri,nice,comm

  PID  %CPU PRI NI COMM
57081  77.6  12 10 cpu-chew
57080  77.1  12  0 cpu-chew
57085  76.9  12 15 cpu-chew
57082  76.9  12 13 cpu-chew
57083  76.7  12  0 cpu-chew
57031   0.0  31  0 -tcsh

And further tangential stuff:

tropic: cat cpu-chew.c

#include <math.h>

int main() {
    double i=2;
    while (i != sqrt(i)) 
        ++i;
    }

Running limit doesn't show anything odd.