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According to Ilya Grigorik who works on Google Search Engine , redirects kill performance especially on mobile , I dont understand why ??

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Because of the nature how redirects work, there are 2 requests made during a redirect.

The example will use redirected CSS file (from example.com/content.css to example.net/some/folder/content.css), but it works same way for all redirected content:

  1. The initial request to fetch example.com/content.css.
  2. The response for this request will be that it is redirected to example.net/some/folder/content.css.
  3. The browser will fire another request to example.net/some/folder/content.css.
  4. The browser will receive content.css.

Now, without redirection, this process would only need 1 request to process:

  1. The initial request to fetch example.com/content.css.
  2. The browser will receive content.css.

As there might be many resources redirected on a single web-page, you can see how that will impact performance as it needs to make two requests instead of one every time. Using Permanent Redirects helps here for clients as browser caches the redirection and should issue requests to redirected content straight away. This will not be the case for Google Crawler I assume, as it should really check raw (non-cached) requests every time too track changes in the redirects.

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If you redirect to a different domain you also have the overhead of another DNS lookup. – DisgruntledGoat Jun 11 '14 at 9:27

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