I've been wanting to write this post for a long time. It's something I've been thinking about and researching for quite a while now.

We're starting to see more and more evidence that page load speed can have a significant impact on conversion rate and visitor perception.

But it's hard to get an accurate understanding of the impact. Most of the published information on this topic is far from unbiased (I link much of the published info below).

Likewise, most (all?) of the scientific literature consists of studies focused on participants' attitudes rather than experiments on actual conversion numbers.

So reliable data on what a typical business should do is sparse.

With that in mind, we have to make decisions on how to prioritize site speed, so we have to use the data we have to make those decisions. To that end, I put together these general guidelines based on the available data that I've linked to below.

  • Studies have shown that a faster site is perceived as being higher quality than a slower site. So it's not just about conversion rates or traffic, it's also about how potential customers view the site, and by extension, the business.
  • Based on the available data, I think we can confidently say that site speed definitely has a direct impact on conversion rates and revenue. Exactly how big of an impact is much harder to determine (keep a later bullet for more).
  • We also know that people are becoming more accustomed to faster sites and using their mobile devices more and more.
  • We know more people are focusing more and more effort on site speed and so the speed needed to remain competitive will probably keep dropping.
  • Most of the data we have available outside of scientific literature is from companies where it makes sense that users would prefer ultra fast page load times (e.g. search engines and consumer ecommerce). It's unlikely that the impact will be as strong on your site.
  • While the evidence isn't overwhelming, I think it does point in the direction that we should shoot for <= 2 second load times as a general rule. If you're not there, consider prioritizing site speed.
  • Shoot for about 2 seconds or less for really good performance.
  • If you're above 5 seconds, this should probably be a major priority.
  • I think, for most companies, using about 5% conversion rate improvement per second (half of what Greg Linden found at Amazon below) is a good rough guide for estimating ROI. Correct a bit for specific situation. So we can estimate a 5% increase in conversion rates with every 1 second improvement in average site speed. In other words, an ecommerce site doing $5,000,000 revenue per year would see a $250,000 annual return for every 1 second improvement in speed.
  • It's probably a good idea to make site speed a major priority if we have load times over 5 seconds.
  • Purely anecdotally, search rankings do appear to be very sensitive to relatively small changes in user behavior, so site speed could actually have a greater percentage impact on search traffic than on-site metrics. Because of that, use 2% per 100-milliseconds for estimating ROI. So even though you might only see a 10% conversion rate improvement from site speed improvements, you might see a much larger increase in search traffic.

Sources for further reading

  • Akamai studied online retailers and found that a 100-millisecond delay in page load time can hurt conversion rates by 7%. That's one-tenth of a second. (Note: This company sells website performance services, so I actually think this is the least reliable information).
  • Marissa Mayer (former Google VP) 2008 Keynote at Google IO: Searches and revenue from search dropped 20% with a half second increase in load time of search results.
  • Some pretty established evidence in scientific journals that show users respond more favorably to faster sites. While credible, these all rely on self-reported measures of attitudes toward a site rather than direct measures of conversion rates, revenue, etc.
  • Greg Linden from Amazon, in a presentation he gave at Stanford titled, "Make Data Useful," said a 100-millisecond delay in page load time costs Amazon 1% of revenue.
  • Eric Schurman (Bing) and Jake Brutlag (Google Search) co-presented results at O'Reilly's Velocity 2009 Conference from latency experiments conducted independently on each site. Bing found that a 2 second slowdown changed queries/user by -1.8% and revenue/user by -4.3%. Google Search found that a 400 millisecond delay resulted in a -0.59% change in searches/user. What's more, even after the delay was removed, these users still had -0.21% fewer searches, indicating that a slower user experience affects long term behavior.
  • Phil Dixon, from Shopzilla, at that same O'Reilly conference, shared that a 5 second improvement in site speed (from 7 seconds to 2 seconds) resulted in a 25% increase in page views, a 7-12% increase in revenue, and a 50% reduction in server hardware costs.
  • Another good resource largely focused on latency but with some links to page speed related sources.
  • Google research found that probability of a user bouncing increases 32% as page load time goes from 1 second to 3 seconds, and 90% as page load time goes from 1 second to 5 seconds.
  • Google also found that if people having a negative mobile experience on a site are 62% less likely to purchase in the future.
  • Finally, Google has this calculator that can help you estimate ROI of page speed improvements based on their data.