Hello,
patch is very simple – utilizes dupstring's internal strlen to not call
strlen again outside. Following test function:

strtest() {
    a=""
    i=$(( 20000 ))
    while (( i -- )); do
        a+="aaaaaaaaaaaaaaaaaaaaaaaaa"
    done
}

runs for 1511 ms with optimizations, 1829 ms with no optimizations.
Changing to i=$(( 10000 )) gives times 364 ms 414 ms, 50 ms difference
(minimum obtainable times).

-- 
  Sebastian Gniazdowski
  psprint@fastmail.com