jtescher/carrierwave-imageoptimizer — pull request #1 by bmayes (2013-04-03 20:47:23+00:00)
Title: Removing the call to manipulate! improves Jpeg compression performance. ...

... The writing back by rmagick of the image after the compression step caused an increase in image size:

write_block = create_info_block(options[:write])
if options[:format]
frames.write("#{options[:format]}:#{current_path}", &write_block)
else
frames.write(current_path, &write_block)
end

It seems like PNGs are unaffected by this change This only improves Jpeg.