Premailer performance

Question

I am using foundation as front end framework,premailer for rendening inline css in my RoR project.The email content i generate in my application are huge.It takes lot of time to convert CSS inline.

class Premailer
  module Adapter
    module Nokogiri
      def to_inline_css 
        ....
        doc.search(selector).each ... 
        .....
      end
    end
  end
end

doc.search(selector) is super slow for huge content in doc.

It takes about 15 -20 seconds to render http://foundation.zurb.com by making CSS inline in http://premailer.dialect.ca/.

Please suggest an efficient way to convert HTML pages with inline css