| time bash -c 'D=1; while true; do if [ "$D" -lt 1000000 ]; then D="$((D+1))"; else break; fi; done'
bash -c   6.44s user 0.00s system 99% cpu 6.447 total
| time zsh -c 'D=1; while true; do if [ "$D" -lt 1000000 ]; then D="$((D+1))"; else break; fi; done'
zsh -c   4.18s user 1.28s system 99% cpu 5.463 total
| time dash -c 'D=1; while true; do if [ "$D" -lt 1000000 ]; then D="$((D+1))"; else break; fi; done'
dash -c   1.95s user 0.00s system 99% cpu 1.950 total

При этом можно чутка подзабить на posix и выиграть времени, но до dash оно не дотягивается

| time bash -c 'D=1; while true; do if [[ "$D" -lt 1000000 ]]; then D="$((D+1))"; else break; fi; done'
bash -c   5.33s user 0.00s system 99% cpu 5.336 total
| time zsh -c 'D=1; while true; do if [[ "$D" -lt 1000000 ]]; then D="$((D+1))"; else break; fi; done'
zsh -c   2.64s user 0.92s system 99% cpu 3.561 total