在Nokogiri中选择子节点的绝对最便宜的方法是什么？

I know that there are dozens of ways to select the first child element in Nokogiri, but which is the cheapest? I can't get around using Node#children, which sounds awfully expensive. Say that there are 10000 child nodes, and I don't want to touch the 9999 others...I know that there are dozens of ways to select