For comparison, PHP has mutable strings. Some totally unscientific averaged benchmarks (Atom D2700, PHP 5.4.4-14 amd64):

    $buff = '';
    for ($i = 0; $i != 100000; ++$i) {
            $buff .= $i;
    }

Runs in about 0.06s

    $buff = array();
    for ($i = 0; $i != 100000; ++$i) {
            $buff[] = $i;
    }
    $ret = implode('', $buff);

Runs slower, in about 0.10s

    $ret = implode('', range(0, 100000));

Takes roughly the same time, 0.10s