Rails core - Feb 2014 - X-Runtime header doesn't seem to be reliable

As far as I understand, Rails uses a middleware by default that will 
send the total time spent on a request in the Rails side in the 
X-Runtime HTTP header.

But it doesn't seem to be reliable in the sense that when I perform a 
request against http://localhost:3000/ (development environment, tested 
with both Puma and Webrick), I get 10ms of latency when serving some CSS 
assets but 109ms of latency for the index page. When I look at the 
header for the same request, I get "X-Runtime: 0.034849".

Here's how it reports the timing in the server logs:

Completed 200 OK in 22ms (Views: 1.4ms | Sequel: 13.2ms)
   Rendered search/_fields_finder.html.erb (0.0ms)
   Rendered search/_left_pane.html.erb (0.7ms)
   Rendered search/_index.html.erb (8.9ms)
   Rendered transactions/_list.html.erb (0.0ms)
   Rendered search/_saved_searches_dialogs.html.erb (0.2ms)
   Rendered search/_saved.html.erb (0.5ms)
   Rendered main/_change_log.html.erb (0.1ms)
   Rendered main/_glossary.html.erb (0.0ms)
   Rendered themes/default/_footer.html.erb (0.1ms)
   Rendered main/index.html.erb within layouts/main (70.5ms)

This happens because I'm using "render stream: true" in that
action, but
I'd expect the latency to be reduced when looking at the Chrome reported 
timing, but it doesn't matter if I render with "stream: true" or
not,
the latency reported by Chrome is always about 100ms.

Am I missing something?

Thanks in advance,
Rodrigo.

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