this is something I see people do a lot

rendering_benchmark.rb

require "rubygems"
require "benchmark"
require "action_view"

obj = {
  :stuff => %w[ ant bee cat dog eel fly gnu ],
  :yeah  => {
    :nacho => "cheese"
  },
  :junk => [12,2312,223,33,434,45,454,345345,213,32,32,45,45,54,45,456,56,56,234,34,34,56,56,56,56,56,5677,567,234,34,234,345,345,435,345,3456]
}
av = ActionView::Base.new

N = 30_000
Benchmark.bmbm do |x|
  x.report("render :json => obj        ") do
    N.times do
      av.render :json => obj
    end
  end

  x.report("render :json => obj.to_json") do
    N.times do
      av.render :json => obj.to_json
    end
  end
end

__END__
Rehearsal ---------------------------------------------------------------
render :json => obj           0.210000   0.010000   0.220000 (  0.217348)
render :json => obj.to_json  21.300000   0.070000  21.370000 ( 21.368202)
----------------------------------------------------- total: 21.590000sec

                                  user     system      total        real
render :json => obj           0.210000   0.000000   0.210000 (  0.214650)
render :json => obj.to_json  20.880000   0.070000  20.950000 ( 20.949253)