Since 2010 Google's ranking algorithm has taken into account how quickly your website loads, when comparing to websites similar to yours. In other words, all things being equal, Google favours presenting faster loading sites in search results than slower ones.
Optimizing your images using Kraken.io can go a long way to getting your pages loading a lot faster, thereby improving your search ranking and overall customer satisfaction through improved usability.
Even if loading speed were not a factor in terms of search ranking, studies have shown time and time again that page loading time can have a drastic impact on conversion rates and your bottom line, for the seemingly obvious reason that users simply hate waiting around for things to load. Below is just a taste of what to expect of users these days (source: kissmetrics)