People say "my site is slow" and almost always mean the same thing: it is slow on mobile, on the connection you have when you are out somewhere. That is also where it matters most — that is where most visitors are.
Here is what it usually turns out to be, in the order we actually find them, not the order the textbook lists them.
1. The images (nine times out of ten)
Someone took a photo straight off their phone and uploaded it. It is 4,000 pixels wide and weighs 6 MB. The browser shrinks it to 400 pixels on screen — but it downloads the whole file first.
Five of those on the front page is 30 MB. On mobile that is not a slow site. That is a site people leave.
Check it yourself: right-click an image, save it, look at the file size. Over 300 KB for an ordinary image is too much. Over 1 MB is a mistake.
2. Fonts fetched from elsewhere
Most sites pull their fonts from Google. That means the browser has to talk to an entirely different server before anything readable appears. You often see the text jump when the real font finally lands.
The fonts can sit on your own server instead. Then they are there immediately — and as a bonus you are not sending your visitors' IP addresses to Google, which is probably what your privacy policy already claims you do not do.
3. Everything that listens in
Google Analytics. A Facebook pixel. A chat widget. A third-party cookie banner. A reviews box. Each one is a small program fetched from a stranger's server and run in your visitor's browser.
Each looks harmless. Five of them is half a second before your page even responds to a click.
Check it yourself: right-click the page, pick "Inspect", go to the Network tab, reload. Now you can see everything being fetched and where it comes from. It is often an eye-opener.
4. The template that does everything
A template has to sell to thousands of different businesses, so it carries everything: galleries, animations, sliders, ten page types. You use maybe a tenth of it. The rest still downloads.
That is why a template site rarely gets properly fast, no matter how many images you compress. The foundation is heavy.
5. The server
The cheapest hosting shares one machine between hundreds of sites. When the neighbour's shop is busy, yours gets slow. If the server sits in the US, every request crosses the Atlantic and back.
It is last on the list because it is the rarest cause — but when it is the cause, nothing else helps.
What it costs you
Google has used page speed as part of ranking for years. Two pages with the same content, one of them fast: the fast one sits on top.
And people do not wait. A visitor who clicks back after three seconds is a customer you never knew you had.
Check it in two minutes
Search for PageSpeed Insights, paste your address, and look at the mobile score, not desktop. Under 50 is a problem. Over 90 is how it should be.
The tool also tells you what is dragging it down. The first three things on this list you can fix yourself.