All frontend servers do are move bits from one place to another -- from backend to client or from disk to client. This does not require much horsepower.
(SSL is probably the biggest drain on resources, as it is the only part of the chain that is doing actual compute work. I have never run a big site with SSL, though, so I am not sure how this works out in reality.)
I think you might be mistaking our load balancers with our frontends.
Our frontend servers run the Rails application (as well as other applications like the Sinatra app running http://jobs.github.com). Additionally, they also run all of our background jobs.
