The basic principle of search engines is to give users the best possible experience so they don't wander off and start using another search engine.
Giving people excellent, relevant content is one part but giving it to them fast is another. If a search engine comes to the conclusion that two pages on two different websites are broadly similar (both have similar content, similar backlinks, etc.) but one is grindingly slow to load then that snail gets ranked lower.
In reality there are billions of websites out there and thousands competing for various indivdiual topics that you or I might search so slow websites tend to get pushed far down the search results even if their content might be slightly better.
In this day and age we are no longer talking about websites that load in 10 seconds versus those that load in 2. Loading times can be almost invisible to the naked eye but to a search engine crawler they are absolute figures.
Be aware that page load speed is a small factor in SEO unless your pages really are taking half the day to render on a users browser. if your pages are loading reasonably fast (and we'll see how to check this in a moment) then just because your page loads 0.1 of a second faster than a competitor doesn't mean you should be ranking ahead of them. Other SEO factors carry greater weight.
There are a number of tools out there where you can compare your site with those of your competitors. I personally use Pingdom's Website Speed Test as it breaks down what the speed at which individual parts of your page load and that can help you identify issues.
You can also sign up for membership which allows you to monitor speed over time, identifying certain days or hours where your site might be suffering. Slightly unrelated but useful, it also monitors to see if your site crashes on a regular basis (another SEO no no) which could be down to poor hosting or coding errors.
Google also offers its own PageSpeed Insights tool but use this with care as it does not actually measure your page loading speed but rather goes through a checklist of things which might affect your page loading speed.
If you see that, compared to your competitors, your loading time is poor then taking action is all part of Search Engine Optimization. It's not a silver bullet, if your page loads twice as fast as your competitors it doesn't mean you will rank higher. This is just one of many factors but one that must be addressed.
So what can you do?
If you have got this far in the guide you should have cleaned up your coding by now. If you have arrived here via a search engine then have a look at:
In short, make sure your code is an clean as a whistle and that there is nothing unnecessary in there.
There shouldn't really be any need for any image to be over 100kb in size. Small icons might not even reach 1kb. If they do use a graphics package such as Photoshop to reduce the image size while keeping the visual quality.
Hopefully as covered in Domain name and hosting in SEO you will have chosen a reputable host for your site. If you're not sure create a blank page on your site and test that through Pingdom or search around the web to see if other people are having issues with your host.
You can add code to your page which tells browsers what to remember and for how long. This way someone visiting several pages on your site, for example, does not need to reload your logo every time they open a page. If you tell browsers to remember your logo for a week that same person coming back tomorrow will not need to wait while your logo is downloaded.
As an example part of a CSS file might look like this:
Minified it looks like this:
This Google Developer page has some handy links for online tools to help you minify your content.
Services like CloudFlare can dramatically reduce your loading times by saving some of your content temporarily in the memory of their super fast servers.
I'm Tim Hill, a Search Engine Optimisation and Online Marketing specialist. I created this site to help others understand that SEO is not a mysterious black art!.
If your a newbie try the Getting Started in SEO page, otherwise feel free to dig around and learn more.