What is SEO optimization?
SEO optimization, or simply SEO, is a collection of practices for helping search engines determine what kind of information is offered on a website, and by extension, enabling users who are looking for that information to find it. SEO stands for 'search engine optimization.'
In practice, SEO basically boils down to publishing quality content and then making the most of that content by aligning the text with the way users are searching for that kind of content. It's much more than simply stuffing content full of keywords*; SEO-optimized sites have clear and direct titles, helpful section headers, descriptive file names and alt tags for images, and easy-to-understand site hierarchies.
From a search engine's perspective, it is important to make it easy for users to find the information they need, so a website's user experience is very important. Site performance is a big part of user experience; it is frustrating for the user when they have to wait for pages to finish loading.
*Keyword stuffing is the practice of using keywords multiple times in content to an unnatural degree in an attempt to gain an advantage in search engine rankings. Google no longer considers the frequency with which keywords are used to be relevant and sometimes penalizes sites for clear instances of keyword stuffing.
What are search engine rankings?
When a user performs a search using a search engine, the search engine generates a list of websites that are put in order from most likely to least likely to fulfill the query. This list is typically divided into a series of results pages (or SERPs) listing websites that may answer the user's query, and usually there are 10 results per SERP. Most users will not go past the first page of search results, opting instead to refine their search and try again. For this reason, the goal of most SEO experts is to get the website they are managing into the top 10 search results for the queries they are targeting.
A website will rank higher or lower in search engine results depending on a number of factors, including how relevant it is for the query and how authoritative the search algorithm considers the website to be. By optimizing a website for search, website owners can make sure their content shows up as high as possible in search rankings, making it more likely that users will click.
In the West, Google dominates the search engine market, and because of this Google's algorithms are largely what determine SEO best practices. In North America, Google has close to 90% of all searches, with Bing and Yahoo search taking up most of the rest of the market. However, in other parts of the world, other search engines hold sway; for instance, Baidu is the main search engine in China.
How much does site speed affect SEO?
In 2010, Google announced that site speed would be factored into their search rankings. Since that time, site performance has grown more influential, and optimizing for performance is now a large part of optimizing for SEO. In their initial announcement of site speed as a ranking factor, Google emphasized the perspective of the user, and how a slow website adversely impacts the user's experience. Google also pointed out that users were less likely to stay on websites that loaded slowly. (This would imply that users were therefore less likely to stay long enough to get their query answered.)
Google does not reveal details about its search algorithms, so it is difficult to quantify exactly how much site speed affects SEO. However, website usability and the overall user experience are certainly crucial, especially for mobile devices, and Google's own research has shown that 53% of mobile sites are abandoned altogether if they take longer than 3 seconds to load.