It's obviously very important for Google to be instrumenting and logging performance. But considering the simplicity of their search UI, I'm not sure why query proc times are still exposed. I also can't find any other examples of search that have this behavior, including direct competitors (Bing, Yahoo, duckduckgo, etc.).
You would think that Google being a huge company would have good solid reasons for everything they do. But building software (and search engine websites are software) involves billions of small decisions taken by each engineer involved. It's everything down from "should I name this temporary variable 'i' or 'n'?" to "should the query time be displayed?" and "what format? milliseconds or fractions of a second? should it be i18n:ized?"<p>My theory is that it just happened because some engineer decided to add it a long time ago and then noone found a good reason to remove it.
Speed was one of the key factors that set Google search apart from the competition in the early days. So at that time they wanted to highlight it. Everyone thought "wow how are they searching so fast?"