Tweak Postgresql config to your liking on Rails boot. Works on Heroku.

config_initializers_database.rb

Rails.configuration.after_initialize do
  case ENV['DB_SPEED']
  when 'fast'
    ActiveRecord::Base.connection.execute "update pg_settings set setting='off' where name = 'synchronous_commit';"
  when 'safe'
    ActiveRecord::Base.connection.execute "update pg_settings set setting='on' where name = 'synchronous_commit';"
  end
end