Why it takes so long on Proc#call and API::Root#call

I am using Grape on top of Rails 4.2.1 to provide API for our application.

But when I check Newrelic for performance today I found that RackApp Proc#call and Grape API::Root#call are taking up large amount of time. (See the screenshot)

What actually are they doing? I figured that Grape API::Root#call is what my code is doing inside an API block, but what's RackApp Proc#call doing and why it takes up so much time? How could I know that?

Thanks a lot for the help.