Hello
The function contains arrdup() that includes elements beyond end index.
I've replaced it with arrdup_max() that has limit parameter – will
duplicate at most that limit of elements. Following test code:

test_fun() {
    local -a arr
    repeat 12; do
        arr+=( "abcdefghijklmnop" $arr )
    done

    local -a arr2
    repeat 2000; do
        arr2=( ${arr[1,300]} )
    done
}

generates array of 4095 elements, and the running times are 1099 ms
(optimized) vs 2038 ms (unoptimized). The array generation utilizes
previous array optimization.

More, I suspect a memory leak in following code that has been replaced:

    if (v->end <= v->start)
        s[0] = NULL;
    else if (arrlen_ge(s, v->end - v->start))
        s[v->end - v->start] = NULL;

That code adapts array according to end index – however it seems that
strings after the NULL are then unreachable to freearray() ? That said,
I wasn't able to obtain the memory leak with repeated ${arr[1,2]} use
for $#arr > 2.

Interesting that some tests fail (e.g. ./Y03arguments.ztst) if I here
duplicate nular instead of doing:

    } else if (v->end <= v->start) {
        s = arrdup_max(s, 1);
        s[0] = NULL;

like in the original code. The test output is then:

@@ -6,6 +6,6 @@
 line: {tst r}{}
 line: {tst x}{}
 line: {tst x }{}
-MESSAGE:{no more arguments}
+MESSAGE:{}
 line: {tst x y }{}
-MESSAGE:{no more arguments}
+MESSAGE:{}

-- 
  Sebastian Gniazdowski
  psprint@fastmail.com