The Page Speed Tax: How Slow Load Times Quietly Kill Conversions
Nobody closes a slow-loading tab and thinks "well, that cost the business money." They just leave. That's what makes page speed one of the most underrated levers in CRO: the damage is invisible in your analytics unless you're specifically looking for it.
Portent ran a study across seven sizeable websites, spanning automotive, education, SaaS, and travel, averaging 764,000 monthly sessions and $4.6 million in monthly revenue between them. The finding held across the board: as First Contentful Paint got faster, revenue potential went up, and a stronger Cumulative Layout Shift score correlated with more goal completions.
The numbers get more concrete the more you dig
A widely cited piece of Google-collaborative research found that a 0.1 second improvement in load time was associated with a 10.1% increase in conversions for travel sites, 8.4% for ecommerce, and 3.6% for luxury retail. Separately, the common industry benchmark is that conversion rates drop by roughly 4.42% for every additional second of load time between 0 and 5 seconds.
And this isn't new. Amazon's internal research, now something of an industry legend, found that a 100 millisecond delay could cost about 1% in sales. That's been repeated so often it's easy to dismiss as folklore, but the more recent, more rigorous studies keep landing in the same neighborhood.
Why this gets deprioritized anyway
Speed work is unglamorous. It doesn't produce a headline like "we redesigned the hero and lifted signups 12%." It's compressing images, deferring scripts, fixing layout shift caused by a font loading late. None of it photographs well in a case study deck.
But it compounds across every single page view, every single visitor, all the time, with no test needed to prove the direction is right. The only open question is how much it's worth on your specific site, and that you can only find out by measuring your own Core Web Vitals against your own conversion data.
Where to look first
Cumulative Layout Shift is the easiest win most sites ignore: it's usually caused by images without dimensions set, ads or embeds loading late, or web fonts causing a visible text jump. First Contentful Paint is usually a hosting and asset-loading problem, not a design problem, which is why a page can look fast in a screenshot and still be slow in reality.
If you want a straight read on where your own site is leaking conversions to load time, that's part of what we cover in a free consultation.