Most companies active on the web understand that speed matters and that slow can kill sales.

But while the impact of a slow website generally is well-established, there’s far less data about the quantitative impact of performance when it comes to one of the increasingly important content types on the web — video. Until now.

According to a study (PDF) conducted by computer science professor Ramesh Sitaraman, user expectations are equally high for online video. In looking at some 23m video views generated by 6.7m unique users, Sitaraman found that users, not surprisingly, have no qualms about abandoning a video if it takes too long to play.

Just how long is too long? By five seconds, 20% of users will leave for short-form video; purveyors of long-form video have a few seconds of wiggle room. All told, each second of delay after the first two seconds increases the abandonment rate by nearly 6%.

For slower websites, the news gets worse: as has been established with slow websites generally, a single bad experience can have a lasting negative impact. “A viewer who experienced a failed visit is less likely to return to the content provider’s site to view more videos within a specified time period than a similar viewer who did not experience a failed visit,” Sitaraman explains. And the reduction is “significant.”

Higher connection speeds bring higher expectations

One of the most interesting findings of Sitaraman’s study is the fact that “Viewers watching video on a better connected computer or device have less patience for startup delay and abandon sooner.” Mobile users, for instance, were seen to be far more patient and willing to tolerate playback delays than their better-connected counterparts.

For publishers seeing mobile usage skyrocket, that may seem to provide some relief, but that relief might be short-lived. Another study published at the beginning of the year revealed that in 2011, 71% of those surveyed expected a site to load just as quickly on a mobile device as on a desktop computer, up from just 58% in 2009.

As mobile networks get faster, it seems logical that expectations for mobile performance will rise too — including for video. So the message to publishers is clear: faster is better, and as consumers see the speed limit increase for both their wired and wireless connections, the bar will continue to be raised for what constitutes ‘fast.’