Frankly, if you're going to be processing requests and shuffling them to S3, Ruby is nowhere near being the limiting factor unless you do something pathologically bad.

Having spent time recently optimizing a setup that does pretty much exactly what they're doing (in Rails; I don't like Rails, but in this case Rails is not a problem), the time saved doing things like eliminating unnecessary buffering all over the place (e.g. POSTs gets buffered by pretty much everything that likes to consider itself a web server, sometimes multiple times; if you e.g. run Nginx in front of pretty much any Ruby web servers running Rails, worst case you may end up with things passed through at least 3, possibly 4 buffers).

Basically they should be IO bound.