Faster version of genetic algorithm

gistfile1.txt

Original code:
perl6-BitVector,16,5.34862844
perl6-BitVector,32,6.211466
perl6-BitVector,64,7.8329269
perl6-BitVector,128,11.85552583
perl6-BitVector,256,18.9098477

Adapted code:
perl6-BitVector,16,3.3355078
perl6-BitVector,32,3.8888316
perl6-BitVector,64,5.3188300
perl6-BitVector,128,8.07596295
perl6-BitVector,256,13.6082418

Actual adapted code:
    my int $len = 16;
    my int $maxlen = 256;
    my int $how-many =100000;
    while $len <= $maxlen {
        my $start = now;
        loop (my int $i = 0; $i < $how-many; ++$i) {
            my int $start = ($len -2 ).rand.Int;
            my int $this-len = ($len-$start).rand.Int;
            my int8 @chromosome1 = Bool.roll($len);
            my int8 @chromosome2 = Bool.roll($len);
            my int8 @x-chromosome = @chromosome2;
            @chromosome2.splice($start,$this-len, @chromosome1.skip($start).head($this-len));
            @chromosome1.splice($start,$this-len, @x-chromosome.skip($start).head($this-len))
        }
        say "perl6-BitVector,$len,",now - $start;
        $len = $len*2;
    }