The interplay between the response latency of web search systems and users’ search experience has only recently started to attract research attention, despite the important implications of response latency on monetisation of such systems. In this work, we carry out two complementary studies to investigate the impact of response latency on users’ searching behaviour in web search engines. We rst conduct a controlled user study to investigate the sensitivity of users to increasing delays in response latency. This study shows that the users of a fast search system are more sensitive to delays than the users of a slow search system. Moreover, the study nds that users are more likely to notice the response latency delays beyond a certain latency threshold, their search experience potentially being a ected. We then analyse a large number of search queries obtained from Yahoo Web Search to investigate the impact of response latency on users’ click behaviour. This analysis demonstrates the signi cant change in click behaviour as the response latency increases. We also nd that certain user, context, and query attributes play a role in the way increasing response latency a ects the click behaviour. To demonstrate a possible use case for our ndings, we devise a machine learning framework that leverages the latency impact, together with other features, to predict whether a user will issue any clicks on web search results. As a further extension of this use case, we investigate whether this machine learning framework can be exploited to help search engines reduce their energy consumption during query processing.