Rails Application Very Slow To Start After Upgrade to Rails 5.1.3

Question

I run a Rails 5.1.3 application on Pivotal Web Services. Since the upgrade from Rails 4 the application has the following behaviour:

• deployment runs fine
• puma starts
• pivotal container is healthy
• the first client that connects sits waiting for 2 to 3 minutes before being served the first page; during this period the server is using up to 100% CPU

This happens also when a new instance is being instantiated.

I have tried changing the 'eager loading' setting both true and false but no change in behaviour seen.

config.eager_load = true

I have tested with and without the rack-cors gem and also no changes.

Here is my config/application.rb:

require_relative 'boot'

require "rails"
# Pick the frameworks you want:
require "active_model/railtie"
require "active_job/railtie"
require "action_controller/railtie"
require "action_mailer/railtie"
require "action_view/railtie"
require "sprockets/railtie"
require "rails/test_unit/railtie"
require 'pdfkit'

# Require the gems listed in Gemfile, including any gems
# you've limited to :test, :development, or :production.

Bundler.require(*Rails.groups)

module MyFaro
  class Application < Rails::Application
    # Initialize configuration defaults for originally generated Rails version.
    config.load_defaults 5.1

    # Settings in config/environments/* take precedence over those specified here.
    # Application configuration should go into files in config/initializers
    # -- all .rb files in that directory are automatically loaded.

    # Set Time.zone default to the specified zone and make Active Record auto-convert to this zone.
    # Run "rake -D time" for a list of tasks for finding time zone names. Default is UTC.
    config.time_zone = 'Brussels'

    # The default locale is :en and all translations from config/locales/*.rb,yml are auto loaded.
    #config.i18n.load_path = Dir[Rails.root.join('config', 'locales', '*.{rb,yml}').to_s]
    config.i18n.default_locale = :nl

    #config.skylight.probes += %w(mongoid mongo)

    config.middleware.use PDFKit::Middleware, {:print_media_type => true}, :only => %r[/print_story]

    config.after_initialize do
      ApplicationController.helpers.cache_set_look_up_hash
      ApplicationController.helpers.cache_set_system_parameter_hash
    end
  end
end

and my config/environments/production.rb:

Rails.application.configure do
  # Settings specified here will take precedence over those in config/application.rb.

  #CORS
  config.middleware.insert_before 0, Rack::Cors do
    allow do
      origins '*'
      resource '*', :headers => :any, :methods => [:get, :post, :options]
    end
  end

  # Code is not reloaded between requests.
  config.cache_classes = true

  # Eager load code on boot. This eager loads most of Rails and
  # your application in memory, allowing both threaded web servers
  # and those relying on copy on write to perform better.
  # Rake tasks automatically ignore this option for performance.
  config.eager_load = true

  # Full error reports are disabled and caching is turned on.
  config.consider_all_requests_local       = false
  config.action_controller.perform_caching = true
  config.cache_store = :memory_store

  # Attempt to read encrypted secrets from `config/secrets.yml.enc`.
  # Requires an encryption key in `ENV["RAILS_MASTER_KEY"]` or
  # `config/secrets.yml.key`.
  config.read_encrypted_secrets = true

  # Disable serving static files from the `/public` folder by default since
  # Apache or NGINX already handles this.
  config.public_file_server.enabled = true

  # Compress JavaScripts and CSS.
  config.assets.js_compressor = :uglifier
  # config.assets.css_compressor = :sass