Why integer division is faster than bitwise shift in shell?

I'm comparing performance of bash and dash (default sh in Xubuntu 18.04). - I expect sh to be faster than bash - I expect bitwise shift to be faster than division operator. However, I'm getting inconsistencies:

λ hyperfine --export-markdown a.md -w 3 ./*
Benchmark #1: ./calc-div.bash
  Time (mean ± σ):      2.550 s ±  0.033 s    [User: 2.482 s, System: 0.068 s]
  Range (min … max):    2.497 s …  2.595 s    10 runs

Benchmark #2: ./calc-div.sh
  Time (mean ± σ):      2.063 s ±  0.016 s    [User: 2.063 s, System: 0.000 s]
  Range (min … max):    2.043 s …  2.100 s    10 runs

Benchmark #3: ./calc-shift.bash
  Time (mean ± σ):      3.312 s ±  0.034 s    [User: 3.255 s, System: 0.057 s]
  Range (min … max):    3.274 s …  3.385 s    10 runs

Benchmark #4: ./calc-shift.sh
  Time (mean ± σ):      2.087 s ±  0.046 s    [User: 2.086 s, System: 0.001 s]
  Range (min … max):    2.058 s …  2.211 s    10 runs

Summary
  './calc-div.sh' ran
    1.01 ± 0.02 times faster than './calc-shift.sh'
    1.24 ± 0.02 times faster than './calc-div.bash'
    1.61 ± 0.02 times faster than './calc-shift.bash'

| Command | Mean [s] | Min [s] | Max [s] | Relative | |:---|---:|---:|---:|---:| | ./calc-div.bash | 2.550 ± 0.033 | 2.497 | 2.595 | 1.24 ± 0.02 | | ./calc-div.sh | 2.063 ± 0.016 | 2.043 | 2.100 | 1.00 | | ./calc-shift.bash | 3.312 ± 0.034 | 3.274 | 3.385 | 1.61 ± 0.02 | | ./calc-shift.sh | 2.087 ± 0.046 | 2.058 | 2.211 | 1.01 ± 0.02 | Here are the scripts I tested: calc-div.bash

#!/usr/bin/env bash

for i in {1..1000000}; do
    _=$(( i / 1024 ))
done

calc-div.sh

#!/usr/bin/env sh

i=1
while [ $i -le 1000000 ]; do
    _=$(( i / 1024 ))
    i=$(( i + 1 ))
done

calc-shift.bash

#!/usr/bin/env bash

for i in {1..1000000}; do
    _=$(( i >> 10 ))
done

calc-shift.sh

#!/usr/bin/env sh

i=1
while [ $i -le 1000000 ]; do
    _=$(( i >> 10 ))
    i=$(( i + 1 ))
done

This difference is more visible for 5000000: | Command | Mean [s] | Min [s] | Max [s] | Relative | |:---|---:|---:|---:|---:| | ./calc-div.bash | 13.333 ± 0.202 | 12.870 | 13.584 | 1.23 ± 0.02 | | ./calc-div.sh | 10.830 ± 0.119 | 10.750 | 11.150 | 1.00 | | ./calc-shift.bash | 17.361 ± 0.357 | 16.995 | 18.283 | 1.60 ± 0.04 | | ./calc-shift.sh | 11.226 ± 0.351 | 10.834 | 11.958 | 1.04 ± 0.03 |

Summary
  './calc-div.sh' ran
    1.04 ± 0.03 times faster than './calc-shift.sh'
    1.23 ± 0.02 times faster than './calc-div.bash'
    1.60 ± 0.04 times faster than './calc-shift.bash'

As you can see, for both bash and dash, division operator is faster than equivalent bitwise-shift to the right.