YSlow规则之一:Rule 9 – Reduce DNS Lookups提到:
Reducing the number of unique hostnames has the potential to reduce the amount of parallel downloading that takes place in the page. Avoiding DNS lookups cuts response times, but reducing parallel downloads may increase response times. My guideline is to split these components across at least two but no more than four hostnames. This results in a good compromise between reducing DNS lookups and allowing a high degree of parallel downloads.
首先:在浏览器里面加载一个页面,所需要访问的域名数为n,那么完成这次加载就需要而DNS查找通常是blocking call,就是说在查询得到结果之后才能继续,所以越多的DNS查找,反应速度就越慢;


