Pygements を利用してると Jekyll Serve --watch のファイル生成が遅い問題を解決してくれる。 参考 → http://blog.eiel.info/blog/2013/08/30/jekyll-watch-very-slow/

Rakefile

require 'bundler/setup'
require 'thread'
require 'launchy'
require 'directory_watcher'
require 'jekyll'

desc 'preview'
task preview: [:watch]  do

  Thread.new do
    sleep 2
    Launchy.open 'http://localhost:4000/'
  end

  sh 'bundle exec jekyll serve'
end

task :watch do
  options = Jekyll.configuration({})
  source = options['source']
  destination = options['destination']

  dw = DirectoryWatcher.new(source, :glob => Jekyll::Command.globs(source, destination), :pre_load => true)
  dw.interval = 1

  dw.add_observer do |*args|
    t = Time.now.strftime("%Y-%m-%d %H:%M:%S")
    print Jekyll.logger.formatted_topic("Regenerating:") + "#{args.size} files at #{t} "
    sh 'bundle exec jekyll build'
    puts  "...done."
  end
  dw.start
end