Building a performant website that delivers a quality experience to the rapidly growing number of consumers surfing the web on mobile and tablet devices may often be a challenging task, but that doesn’t mean that users are willing to cut companies any slack.

In fact, tablet users expect websites to load in under three seconds, and smartphone users only slightly more patient with a four second expectation.

That’s according to a study published this week by testing and monitoring solutions provider Keynote Systems. It surveyed more than 5,000 individuals about their browsing habits and expectations.

Of those polled who were smartphone users, a full two-thirds indicated that their biggest gripe was slow-loading web pages. What constitutes slow-loading? Well over half (64%) of smartphone users want a web page to load within four seconds; the vast majority (82%) expect it to load within five. Tablet users had even higher expectations: 60% of them want a page to load within three seconds. Other surveys have produced similar results.

Obviously, there are a lot of things that are out of the control of companies when it comes to their mobile and tablet experiences. Users connecting via mobile networks may experience latency due to the quality of their connection; even WiFi connections are susceptible to degradation.

But even though such things are out of a company’s control, it’s clear that users don’t really care which party is responsible for poor performance: according to Keynote Systems, 16% of users will leave and not return if your pages don’t load fast enough. And 6% will leave for a competitor.

These numbers highlight why it’s so important to optimize for mobile and tablet experiences. A mobile or tablet site that looks good isn’t enough. And addressing common issues that could be slowing down your website down may not be enough either. Look at mobile and tablet-specific issues and benchmarking performance on common devices and platforms will arguably become a must to stay competitive and meet user expectations.