Start with the pictures
Oversized images are the number one reason small sites are slow. A photo straight from a modern phone can be several times bigger than any screen needs. Resize pictures to roughly the size they appear, save them in a web-friendly format, and avoid uploading dozens at full size.
Be careful with video and heavy effects
A video that plays behind the first screen looks impressive on a fast computer and painful on a phone. If you want video, let people press play. Be wary of effects that move things around as you scroll: they add code, and can make the page jerky.
Fewer plugins, lighter theme
Every plugin adds some code that every visitor downloads. Keep only the ones you need. Choose a lightweight theme rather than a heavy all-in-one one, and avoid stacking several page-builder tools on top of each other.
Keep fonts simple
One or two fonts is plenty. Each extra font file has to be downloaded before the text looks right.
Check it the way your visitors see it
- Open your site on your phone using mobile data, not Wi-Fi.
- Count how long until you can read the first headline and tap the main button.
- Test your most important page, usually the home page.
- Free speed-test tools can show what is slowing a page down.