On an office computer with fast internet the site opens instantly, and speed seems fine. Meanwhile a client opens it on a phone in the subway, on a weak connection, and sees a white screen, then jumping text, then an image loading from top to bottom.
A slow site is rarely slow because of one big mistake. Usually it is a set of small things, each adding a little. Below is what most often slows a site down on a phone, how to check it yourself, and what to ask the developer.
How to check
Open the site on your own phone over mobile data, not office Wi-Fi. Notice how long it takes for the first meaningful screen to appear, whether text jumps as images and fonts load, and whether you can tap the button right away.
For a more precise picture there is Google’s free PageSpeed Insights. It gives a score and, more importantly, a list of specific causes. Do not make the score the goal: what matters is what a person sees, not a number in a report.
Images
The most common cause. A photo straight from a camera is many times heavier than a phone screen needs. Images should be compressed, saved in modern formats like WebP or AVIF, and served to phones in a smaller version, not the same file a large monitor gets.
- Every image has a width and height set, so the page does not jump
- Images below the first screen load only when the visitor scrolls to them
- The first screen background is no heavier than needed, or replaced with a color
Fonts
Each font weight is a separate file. Regular, bold, italic, an extra bold, and all of it in two typefaces. One typeface with two or three weights is enough. Host the font on your own server and show text in a system font until the main one loads, instead of hiding the text until it arrives.
Scripts and widgets
A live chat, a map, a YouTube video, analytics counters, ad pixels, a reviews widget. Each of these pulls its own code from a third-party server, and the phone has to download and run all of it. Go through the list and ask what is actually used.
What is needed can load later: show the map and the video as a placeholder that loads on tap, and switch the chat on a few seconds after the page loads.
The server and how the site is built
If the server takes a long time before it sends the page, no image compression will help. Cheap overloaded hosting, a heavy CMS with dozens of plugins, and no caching add delay before the browser gets the first byte.
For sites that do not change often, prebuilt HTML pages work well. The server simply sends a file and does not have to assemble the page for every visitor.
What to ask the developer
- How images are compressed and whether they have sizes set
- How many fonts and weights are loaded
- Which third-party scripts are on the site and why each one is there
- Whether the server has a cache and how pages are built
- Whether the site was tested on a phone over mobile data
Speed is easier to build in than to fix later. If the site is already live, start with images and unneeded widgets: that is usually where the most visible result comes for the least effort.
Why images need sizes and what else to check before launch is covered in the note on landing structure. If the site is already live, see what support after launch covers. To have your site’s speed checked, use the contact form.