Is there a good explanations of why Rainbows is better than Unicorn for slow clients? All I seem to have read is that it is better, but without going into detai

Is there a good explanations of why Rainbows is better than Unicorn for slow clients? All I seem to have read is that it is better, but without going into details what is different. We use Nginx in front of Unicorn as suggested, however we also do synchronous file uploads and thumbnailing, the process takes a while (maybe 20 seconds) and I would like to understand whether we should be using Rainbows for this.