If you research page speed online you'll be hard-pressed to find anything other than blog posts and "studies" touting how important it is.

However, unbiased information is nearly nonexistent. As of this writing, there are 2 types of sites1 ranking at the top on Google for "impact of website speed": Companies that sell page speed stuff, and companies that link to them. That's it. So, yeah, nothing reliable. At all.

I'm not saying page speed doesn't matter. It's just that it's damn near impossible to find unbiased info (I link to most of what's available online 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. We do know that page speed has some impact. And it's likely that it always has some impact. But that doesn't help us prioritize the work or know how fast is fast enough.

But part of this job is making decisions with limited data; we know page speed is a thing, some kind of thing, and we need to know how to prioritize it. To that end, I put together these general guidelines based on the available data that I've linked to below, as well as our own experience with client sites.

  • 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 organization behind the site.
  • 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.
  • 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.
  • We can't just generalize the data linked in the studies below, even if we trust it. Shopping on Amazon is not even in the same universe of tasks as, say, finding a fulfillment provider. Amazon may gain 1% revenue for every 100ms of page speed…the fulfillment provider probably won't. So competition, vertical, the nature of the site or app itself, the user, and countless other factors will all have a significant impact on how much speed matters.
  • While the evidence is far from convincing, we have to do something, so I typically recommend starting with <= 2 second load times as a general rule. If you're not there, consider prioritizing site speed.
  • 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) may be a good starting point for estimating ROI. You'll have to correct A LOT for your specific circumstance. A B2C ecommerce site with a ton of competition will probably be a lot closer to Amazon's findings than a B2B manufacturer. If the site's primary job is to sell stuff (i.e. ecommerce), page speed will likely have a much higher impact than if it's just there for information.
  • It's probably a good idea to make site speed a major priority if we have load times over 5 seconds.
  • Google rankings in particular 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. We're using a rough calculation of 5% search traffic per 1 second page load time for estimating traffic increases. I think that's conservative; it might be higher than that. Factor in value per visitor to figure out ROI there.

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.

1 The only exception is Google themselves, which some articles cite as a primary source. But Google also benefits financially from a faster web.