How much does page speed affect conversion rate, and why is mobile lower?
Slow pages lose people before they read anything, and mobile is where it shows up.
Page speed affects conversion rate because a visitor who leaves during load never sees the page at all, and mobile is where that happens most. Rather than quoting somebody's borrowed statistic, here is the mechanism and how to measure the cost on your own site.
Why mobile is worse
Three things stack up on a phone. The connection is slower and less consistent than office wifi. The processor is weaker, so JavaScript that runs instantly on a laptop takes noticeably longer to execute. And the visitor is frequently mid-task: in a parking lot, between meetings, with one hand.
Paid traffic makes it sharper still, because a paid visitor has no prior relationship with you. An organic visitor who found your article will wait a moment. Someone who tapped an ad has four other results behind the back button.
What good looks like
Google publishes its Core Web Vitals thresholds on web.dev, measured at the 75th percentile of real page loads: Largest Contentful Paint of 2.5 seconds or less, Interaction to Next Paint of 200 milliseconds or less, and Cumulative Layout Shift of 0.1 or less. The 75th percentile detail is the important one, because it means three quarters of actual visits must hit the target. A fast test on your own laptop proves nothing about the visitor on cellular.
The usual causes are consistent: oversized hero images, a pile of third party scripts, and fonts that block rendering. Run a page through PageSpeed Insights and look at the field data section, which shows real visitors rather than a lab simulation.
Measure the cost on your own site
You do not need an industry average. Segment your own analytics by device and compare conversion rate on mobile against desktop for the same pages and the same sources. Then look at your Core Web Vitals by device. If mobile conversion is materially lower and mobile LCP is materially worse, you have located both the problem and roughly what it is worth: the gap between the two rates, multiplied by your mobile traffic, multiplied by the value of a lead.
That number is yours, it is defensible, and it makes the business case in a way a headline statistic never will.
Friction beyond speed
Speed is not the only reason mobile converts lower. Tap targets too small or too close together, form fields that do not trigger the right keyboard, autofill broken by custom field names, and popups that cover the content all cost conversions. Fix those alongside the load time.
Related: Why do Core Web Vitals and mobile page speed matter?, Will my website be mobile-friendly?, What is a good conversion rate for a landing page?, and our blog post on adaptive UI and UX.