On today’s Google Webmaster Office Hours, one of the questions was why it can sometimes take significantly longer for Google to process some 301 redirects while others are processed quickly.  John Mueller from Google says there’s actually two parts of this issue.

Google processes 301 redirects right away, as soon as they discover it. So Google will crawl the original URL, discover the 301 redirect, and then crawl the content on the page that you are redirecting Googlebot to and index it.

But sites that have been around for quite some time, especially sites that are large or that haven’t changed for quite some time, can take a lot longer for Google to process the redirect. This is because they tend to not get crawled as frequently as other sites, because Googlebot isn’t seeing any changes on previous visits over time. So this results in a possible delay – sometimes a significant one – between you actually employing a 301 redirect and Google discovering it and processing it.

He also says that using the site: isn’t always the best way to determine whether Google has processed the 301 redirects are not. It is normally someone is searching for that site they will display everything even if they already know it’s moved to a new URL, simply because they know that the person is searching explicitly for results from that site. So using the site: query isn’t the best way to judge what has been done.

He does say if you are doing a 301 redirect, there are some things you can do to ensure the process goes smoothly. You want to follow Google’s site move guidelines, that you have the redirects setting up correctly and using the change of address tool if applicable.

He also recommends checking the index status in Google Webmaster Tools can also help, as you will see the new domain start going up as everything is recrawled while the old domain is going down.