I would try adding the post ID reference to the get_field calls in your function, like:
function perf_pln2($post_id)
{
$total = (get_field('zip',$post_id)+get_field('kraken',$post_id))/(get_field('_price',$post_id)+0.01);
$value = $total;
$field_name = "perf";
update_field($field_name,$value,$post_id);
}
add_action('save_post','perf_pln2');