controller.rb

class Controller
  include LazyLoad

  def show
    @model = Model.find(...)

    respond_to do |format|
      format.html do
        @html_specific_data = Model.find(...)
      end

      render_lazy_load(format) do
        @lazy_specific_data = Model.find(...)
      end
    end
  end
end

helper.rb

module Helper

  def lazy_render(name)
    content_tag :div, :id => "lazy-loaded-#{name}" do
      content_tag :div, :class => "spinner" do
        content_tag(:script) do
          "if(!window.lazyLoads) { window.lazyLoads = []; }; window.lazyLoads.push('#{name}');".html_safe
        end
      end
    end
  end

end

lazy_load.rb

module LazyLoad
  def render_lazy_load(format, &block)
    format.js do
      block.call if block
      render json: {
        lazy_load_partial: params[:lazy_load_partial],
        body: render_to_string(:partial => params[:lazy_load_partial])
      }
    end
  end
end

script.coffee

if window.lazyLoads
  for lazyLoad in window.lazyLoads
    url = window.location.href + "?format=js&lazy_load_partial=#{lazyLoad}"
    $.ajax url: url, dataType: "json", success: (data) ->
      $("#lazy-loaded-#{data.lazy_load_partial}").html(data.body)

view.html.haml

= render 'foobar'

becomes

= lazy_render 'foobar'