What a slow website costs: the research on speed and sales

A tenth of a second sounds like nothing. In large studies and controlled tests, it moved conversion rates by several percent. Why speed is a sales figure, not a technical one.

Speed is easy to treat as a developer's concern, measured in tools nobody in the boardroom opens. The research says it belongs in the sales report.

The thresholds people feel

Jakob Nielsen set out the basic limits in 1993, and they have not changed because human attention has not changed. Around a tenth of a second feels instant. Around one second keeps someone's train of thought intact, though they notice the wait. Around ten seconds is the limit for keeping their attention on the task at all; beyond it, people drift to something else.

A tenth of a second, measured in revenue

In 2020, Deloitte Digital published a study commissioned by Google, using data provided by Fifty-Five, that tracked retail, travel, luxury and lead-generation brands in Europe and the US over four weeks. It looked at what happened when mobile sites became 0.1 seconds faster across a set of speed measures and across the whole visitor journey.

For retail sites, conversions rose by 8.4 percent and average order value by 9.2 percent. Travel sites saw conversions rise by 10.1 percent. Luxury brands saw 8.6 percent more pages viewed per session, and on lead-generation sites the bounce rate on information pages improved by 8.3 percent.

A controlled test

Studies across many brands leave room for doubt, so the cleanest evidence comes from experiments. Vodafone ran a split test on a landing page that received paid traffic: half of visitors saw the existing page and half saw a version optimised for speed, with identical visuals and features. The faster version showed its main content 31 percent sooner. It produced 8 percent more sales, a 15 percent better lead-to-visit rate and an 11 percent better cart-to-visit rate.

Why speed reads as quality

Speed also feeds the first impression. A page that assembles itself in front of a visitor, with text jumping as images arrive, looks unfinished during exactly the moments when people are deciding whether to stay. Fast sites feel more confident, and that confidence transfers to the business behind them.

Where to start

• Measure real visitors, not just a test on a fast office connection. Field data shows what customers on phones actually experience.
• Make the main content arrive first. The largest image or headline is what people wait for; prioritise it and defer the rest.
• Be ruthless with third-party scripts. Chat widgets, trackers and embeds each add delay, and few earn it.
• Serve images in modern formats at the size they are shown.
• Treat speed as ongoing. Every new plugin, tag and campaign can quietly undo earlier gains.

If a tenth of a second can move conversions by several percent, the gap between a slow site and a fast one is not a technical detail. It is revenue you are choosing not to collect.