Working example of SSE using Sinatra and Redis, but with bad concurrency...please help to improve

sse.rb

require 'redis'
require 'sinatra/base'

class SSE < Sinatra::Base

  def send_message(json)
    "id: #{Time.now}\n" +
    "data: #{json}" +
    "\r\n\n"
  end

  get '/channels/:id/subscribe', provides: 'text/event-stream' do
    channel_id = params['id']
    stream(:keep_open) do |connection|
      Redis.new.subscribe("channels:#{channel_id}") do |on|
        on.message do |channel, json|
          connection << send_message(json)
        end
      end
    end
  end

end