The person building the site sits in front of a large monitor; the visitor holds a small screen. The problems that arise from that gap rarely look like faults. The page opens, the text is there. It is just slightly harder to read, the buttons are slightly harder to hit, and people quietly leave.
Start by checking the actual split for your own site rather than trusting a general claim — your analytics has a device report. For most businesses that rely on search traffic, the number leans towards mobile more than owners expect.
The first five things to check
Pick up a phone and walk your own site as a first-time visitor would, in this order.
Horizontal scrolling is both the most common and the least visible of these. The cause is usually a single fixed-width element — a wide table, an oversized image, a long unbroken URL string. One is enough to push the whole page sideways.
Touch targets are equally invisible with a mouse. Links sitting close together mean a finger hits the neighbour. Check the spacing in footer link lists and menu items in particular.
It is not only about screen size
Mobile is not a small desktop. The connection and the circumstances differ too.
This is why image weight is felt far more sharply on mobile. One oversized photograph you never noticed on a desktop becomes the reason the first screen takes its time — the next part deals with that specifically.
One more: be careful with pop-ups. What covered part of a desktop window covers the entire phone screen, and if the close button has drifted off the edge, the visitor’s only remaining option is the back button.
Checking with browser tools
A phone is not always to hand, so use the browser’s device mode as well. Open developer tools, switch to a mobile size, and most layout problems reveal themselves immediately.
Device mode cannot tell you everything, though: real touch accuracy, real connection speed and readability in bright daylight are all outside it. Look at the site on an actual device at least once before launch.
Articles on mobile performance live in the performance archive, and if your site is specifically slow on phones, measurement and improvement run together in our optimization program.
Next part
The heaviest thing on a mobile page is usually a photograph. The next part covers the thirty seconds before you upload one: size, format and filename.