Why integer division is faster than bitwise shift in shell?

Question

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

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

calc-shift.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.