Response Time: Is Speed the Ultimate Usability Metric?

Despite our collectively dim memories of the rough and tumble days of dial-up, you would be hard pressed to find someone willing to wait more than a few seconds for a website to load without a sigh at the very least, if not an aggressive eye roll. The negative impact of slow speed on usability ratings is well documented and referred to as “response time,” indicating the shift of responsibility from the once literal speed of the user’s internet connection to the interface itself. Regardless of the origin of a site’s sluggish response time, the user is almost certain to lay blame on the site. As Jakob Nielsen notes in his discussion of website response times, “slowness (or speed) makes such an impact that it can become one of the brand values customers associate with a site.” Or, as Ray Valdes, the Research VP of Gartner puts it, “pretty but slow is worse than ugly but fast.” And perhaps most compelling when making the case for speed to a business with usability issues is the role it may play in search engine optimization (SEO) for Google, where site speed has been a ranking factor since 2010.

As Nielsen discusses, speed is a determining factor in the usability of a site for two reasons: human limitations and human aspirations. The limitations lie in the processes of our working memory, where information quickly decays and disappears without the necessary recitation compounding it into our long term memory. The aspirations lie in our psychological desire for control over a machine. But, most simply, people “engage more with a site when they can move freely and focus on the content instead of on their endless wait.” (Nielsen 2010)

Users’ high attunement to speed is based on response-time limits established by Miller, 1968 and further explored by Nielsen. There are three response-time limits:

  • 0.1 seconds is the limit for the user to feel that the system in reacting instantaneously to their direct manipulation – the only necessary feedback is the display of results.
  • 1.0 second is the limit for the user’s thoughts to remain uninterrupted, although they will notice the delay.
  • 10 seconds is the limit for keeping the user’s attention – in other words, the general point of abandonment.

So what is the measured cost of these response time limits? Shumway (2014) cites these statistics from research conducted by Gomez and Akami:

  • 47% of consumers expect a web page to load in 2 seconds or less
  • 40% abandon a website that takes more than 3 seconds to load
  • 79% of shoppers who are unhappy with website performance are less likely to buy again
  • Conversions are reduced by 7% for every 1 second delay in page response
  • Reducing load time from 8 seconds to 2 results in a 74% increase in conversions

And to drive the point home:


Clearly speed time is having a significant impact on user experience, one that can cost a business significantly. But the consideration of speed in usability is not one of unbridled proliferation. An unexpected turn in the discussion is the reality that sometimes computers can actually be too fast – in these cases, user interface changes should be timed according to a real time clock versus the computer’s execution speed. The ultimate lesson here is never to dismiss speed as a purely functional trait – users care about it a ton, and they will make you pay for its absence. By remembering the importance of speed in the UX analysis of a web interface, usability experts and businesses alike can avoid spending too much time on other details at the sacrifice of something valued so highly.


Nielsen, J. (1993). Response times: the three important limits. Retrieved from

Nielsen, J. (1997). The need for speed. Retrieved from

Nielsen, J. (2010). Website response times. Retrieved from

Shumway, B. (2010). Site speed, mobile usability, and SEO. Retrieved from

Valdes, R. (2015). User experience design: from the web to mobile to social.