The slowest URL router benchmarked there[1] does 10462.0 runs per second, which is just under 0.1ms per run.
I'm not sure what performance is expected of Rails handlers but the first stackoverflow thread[2] that came up on a simple Google search had twitter doing 600 req/s across 180 instances which comes to 300ms per request.
By that math, the slowest URL router costs 0.033% of the request overhead.
Caveat: 300ms/req is slower than you'd like for an interactive website; real websites may have more complicated routing tables than this benchmark. Counter-caveat: 0.1ms is not enough to matter pretty much anywhere except for the stock market.
[1]: https://github.com/c9s/r3#benchmark
[2]: http://stackoverflow.com/questions/373098/whats-the-average-...
Assume Rails has improved hugely since then, 600 req/s over 180 instances is absolutely abysmal performance O_o
I'd expect Rails to clock in well under 300ms per request now, and looking at Basecamp's site[2], that seems to be the case, impressively snappy page loads (if caching the kitchen sink and not hitting live Rails stack then we'd need to look at a different example, but this site has pretty much optimal load time).
[1] https://blog.twitter.com/2013/new-tweets-per-second-record-a...
This + libev/libuv + the Node.js HTTP parser makes a nice little application server.
You shouldn't go replacing your Ruby apps with C but there are many cases were embedding Ruby/Python/etc into your C app just doesn't make sense and this is a good way of bringing nice features from those more webby frameworks to C.
If we consider complexity before everything, we must use PHP to implement an OS. :-p