Why you care: We begin with an end-to-end example of the design (with explicit assumptions), execution, and interpretation of an experiment to assess the importance of speed. Many examples of experiments focus on the User Interface (UI) because it is easy to show examples, but there are many breakthroughs on the back-end side, and as multiple companies discovered: speed matters a lot! Of course, faster is better, but how important is it to improve performance by a tenth of a second? Should you have a person focused on performance? Maybe a team of five? The return-on-investment (ROI) of such efforts can be quantified by running a simple slowdown experiment. In 2017, every tenth of a second improvement for Bing was worth $18 million in incremental annual revenue, enough to fund a sizable team. Based on these results and multiple replications at several companies through the years, we recommend using latency as a guardrail metric.