Inspired by and for use with Jiffy-Web. This "library" provides the same awkward interface as Jiffy-Web in Ruby/Rails land as singleton. Basically you can do mark() and measure() in Rails just as you would in Javascript with Jiffy-Web. Then at some suitable late point in the request loop you can serialize Jiffy-Ruby to JSON and load that into Jiffy-Web.
Then if you have both Firebug and the Jiffy Firebug plugin installed you get a nice visual for your rails performance metrices right next to your javascript performance metrics.