A simple way to monitor your precious app. Put it inside the initializers and log valuable informations.

slow_queries.rb

ActiveSupport::Notifications.subscribe "sql.active_record" do |name, started, finished, unique_id, data|
  Thread.current["active_record_sql_count"] ||= 0
  if data[:name].nil? || data[:name] != "SCHEMA"
    Thread.current["active_record_sql_count"] = Thread.current["active_record_sql_count"] + 1
  end
 end

ActiveSupport::Notifications.subscribe "process_action.action_controller" do |name, started, finished, unique_id, data|
  time = Time.now
  slow_logfile = File.open(Rails.root.join("log", "slow.log"), 'a')
  if Thread.current["active_record_sql_count"] && Thread.current["active_record_sql_count"] > 10
    output = "===Time: #{time} = Too many queries = Number of queries: #{Thread.current['active_record_sql_count']} - started: #{started} - finished: #{finished} - data: #{data}\n"
    Rails.logger.warn(output)
    slow_logfile.puts(output)
  end

  if (data[:db_runtime] && data[:db_runtime] > 5000) || (data[:view_runtime] && data[:view_runtime] > 5000)
    output = "===Time: #{time} = Slow Controller Action = started: #{started} - finished: #{finished} - data: #{data}\n"
    Rails.logger.warn(output)
    slow_logfile << output
  end
  slow_logfile.close()
end