How to trace the rendering response time in Rails?

I am making an API call which on completion gives a summary of how much
time was consumed to make the ActiveRecord Queries to the DB and Render
the views as a JSON Response. I am using Rails 3.2.11 .

Here's a sample of the response time usage:-

Completed 200 OK in 629ms (Views: 354.9ms | ActiveRecord: 112.8ms)

If I add up the times taken to make the DB call and render the Json
response(I am using Json builder), they sum upto:- 467.7ms. Can anyone
please tell me where the remaining time of 161.3 ms(629 - 467.7) could
have been utilized. Is there a gem I can use in my Rails app to track
the same ?