Shorter is better: 15-second YouTube ads beat 20-second ones across all metrics

When it comes to non-skippable YouTube ads, the basic assumption is usually simple: the longer the ad, the more time you have to communicate your message, and the better it should perform. But when we put this assumption to the test in practice, the results told a very different story.

Using DV360, we conducted a controlled experiment in the Czech Republic and Slovakia, comparing 15-second and 20-second non-skippable YouTube ad formats. Both variants ran simultaneously, using identical targeting and budgets. Creative duration was the only variable.

What we discovered was surprisingly consistent across both markets.

1. In the Czech Republic, the 15-second format achieved a 0.27% CTR, while the 20-second variant reached only 0.25%. That’s roughly an 8% higher click-through rate in favor of the shorter format.

2. This difference was even more pronounced in Slovakia. Here, 15-second ads hit a 0.30% CTR, whereas the 20-second format achieved only 0.20%. This represents a 50% higher CTR for the shorter format—a substantial performance gap.

Although both formats guarantee full views due to being non-skippable, their levels of audience engagement are not equal. This is precisely where the “longer = better” intuition begins to fall apart.

Even though users are forced to watch both variants in full, attention behaves differently than raw impressions. The data suggests that while users remain in the ad environment for the entire 20 seconds, their mental engagement drops over time. Those five extra seconds rarely improve message comprehension; instead, they simply prolong passive viewing.

This contrast becomes particularly clear when looking at efficiency. In Slovakia, the CPM for the 20-second format was approximately 12% higher than for the 15-second ads, with no corresponding improvement in performance. The same pattern emerged in the Czech market: the CPM gap reached 26%—nearly a one-third price difference. Longer formats consistently come with a higher price tag, while shorter formats generate better engagement per euro invested.

In practice, advertisers are paying more for a format that delivers lower engagement efficiency. However, it's important to clarify that 20-second ads aren't useless. On the contrary, they still guarantee complete views and can work very well in certain creative contexts.

They are, however, less cost-effective—and efficiency is often what decides the success of performance campaigns.

The key takeaway from this test isn't that one format is universally “better,” but rather that their cost-to-performance ratios differ significantly. The 15-second format consistently achieves a higher CTR across both markets—from a modest +8% boost in Czechia to a striking +50% in Slovakia—while maintaining a lower CPM.

The 20-second format still has its place, but only when the extra time adds genuine value—such as in storytelling, emotional messaging, or explaining a complex concept. Otherwise, those additional seconds fail to deliver value proportional to their cost.

In other words: in a non-skippable environment, buying time doesn't automatically buy attention. You have to earn it.

Written by Julie Baeva