Why does it matter?
How fast your site loads, or the time it takes to respond to a visitors input, has a massive effect on both how long a visitor is likely to remain on your site, and how much effort this visitor will put in to getting information from your site. The slower and less responsive your site is, the more obstacles you are putting in the way of getting your information across to your users.
What causes a slow site
A slow or sluggish site can be caused by a number of factors. This can be split into two main areas: the hosting and the site’s codebase.
The hosting / infrastructure
Good hosting is fundamental to getting a site to run quickly. There are a lot of variables that can go into figuring out what hosting will work best for your site, but you can be fairly sure that the cheap hosting package advertised as an entry-level option is unlikely to give good results – at best, your site will be jockeying for resources with many other websites on a highly congested environment. Ideally you want to be matching your hosting to your site’s code platform and usage needs.
The site’s codebase
Even with the fastest hosting in the world, the performance of your site will rely heavily on the underlying code, code quality and related media that your site runs. Poorly optimised images and huge pages cause nightmares for mobile or bandwidth-restricted users, and excessively complex or poorly-coded themes and modifications can make your site feel like a sluggish mess.
How do you know there’s a problem?
First, test it! Put yourself in the place of a site visitor and try and use your website, ideally on a browser or system that has not visited your site before (so the lack of local caching doesn’t give you a false sense of speed).
Another good rule of thumb is to give your site a quick run through Google’s PageSpeed Insights
It’s not infallible, but you can be sure that a low score on there indicates there’s a fairly poor experience waiting for your site visitors.
What can you do about it?
In the short term, it’s worth investigating if moving to better hosting will improve the situation. Longer-term, a poorly-developed site will need some optimisation. For a problematic site, it may be necessary to look at developing a more optimised site to remove the issues completely.