Timeout with Heroku

[jlerpscher]

I am trying this gem for production on Heroku but after few seconds, I have some timeout requests (it fails with only 15 online users)

2019-04-11T21:28:29.469474+00:00 heroku[router]: at=error code=H12 desc="Request timeout" method=GET path="/fr/foo/bar" host=mywebsite.com request_id=9c3c8364-e483-44ad-9d55-d030fcb10c6b fwd="90.46.112.209" dyno=web.1 connect=0ms service=30000ms status=503 bytes=0 protocol=https

My configuration is pretty simple:

reverse_proxy /\/fr\/foo(\/?.*)$/i, "#{ENV['NEW_APP_URL']}/fr/foo$1"

Am I missing something?