I had been wondering about it for some time now. I have been tracing middleware times on Graphite and could see a stark similarity between the total application runtime and the runtime for ActionDispatch::RailsRouting::Routset
. The graphs have almost been overlapping.
Found a possible explanation in one of the posts made by Luke Ludwig (the guy who wrote Rack Timer).
It will also print out the time elapsed by the actual application's action, which is combined with Rails routing, the final piece of middleware.
Source: http://pulse.sportngin.com/news_article/show/137153?referrer_id=543230
Turns out that Rails routing includes the total time spent by the application, hence a high runtime.
Hope this helps!