Bash (5.2.15):

$ bench () { timeout 60 env -i bash -c "trap 'echo \$i; exit' TERM; i=0; while [ 0 $1 0 ]; do i=\$((i+1)); done"; }; python3 -c "print(round((1 - $(bench "-eq")/$(bench "=")) * 100, 2), '%')"
7.58 % # 5.76 % or 4.68 % or 3.65 %, YMMV

Dash (0.5.12):

$ bench () { timeout 60 env -i dash -c "trap 'echo \$i; exit' TERM; i=0; while [ 0 $1 0 ]; do i=\$((i+1)); done"; }; python3 -c "print(round((1 - $(bench "-eq")/$(bench "=")) * 100, 2), '%')"
13.07 %