As website speed is a new factor that Google uses for one of their ranking factors someone people have been obsessed with making their site faster. There are a few ways you can make your site faster that will be discussed in a future blog post, but for the current time you should not show Google a page that is different from what you show visitors, as this is known as Cloaking and will get your site de-indexed.
What is Cloaking?
Cloaking is a black hat technique and is mentioned in Google Guidelines,
“Make pages primarily for users, not for search engines. Avoid “cloaking,” which is when you deceive your users or show search engines different content than what you show them.”
Cloaking is when you show the search engine spiders a different page to what you show visitors and is done through the HTML code of your website. Cloaking was a trick that some SEOs/webmasters used to do to try and trick search engines for better ranking and also visitors to their site.
For The Search Engines
If you are cloaking for search engines then you could rank for terms that your site would not normally rank, or rank higher for the terms you do rank for which would bring you more visitors. This would be done if: your site was all in flash, you wanted to rank higher for your keyword so you needed to show the search engine spiders more relevant content, but you did not what the text to clutter up your site, or you just wanted to rank for any key term just to bring more traffic to your site.
For The Users
If you are cloaking for visitors then you could show a different meta-tag snippet to the search engines then to visitors, your site could turn out to be porn and have nothing to do with what the user searched for in the search engines, and because you showed a different page to Google, Google would believe your site was relevant for the key phrase the user searched for.