It's well accepted that Ruby is no performance champion. It's execution speed is slower than that of similar scripting languages like Perl and Python, and significantly slower than lower-level languages like C. Ruby was designed to be a quick language to code in and develop for, but not a quick language in terms of its performance.
On the Ruby newsgroup, comp.lang.ruby, Peter Hickman started a thread suggesting "For performance, write it in C." This approach has been traditionally popular for languages like Perl, and is now becoming common with Ruby. One of Hickman's demonstrations yielded over a 100 times speed improvement between some demonstration Perl code and a C implementation, so the benefits are obvious, and as a language with a compiler on nearly every platform, C makes a good choice for a fast, lower-level language with which to write high-performance code.
With Ruby, there are two options available for using C. The traditional way is to create an extension for Ruby in C that's compiled separately and included in, like a library. You can learn how to do this in How to Create a Ruby Extension in 5 Minutes. A new approach, however, is to use the RubyInline library, a library that allows you to write C alongside your Ruby code. For small routines this is significantly quicker than going to the efforts of writing a whole library in C (though not by much), and also 'cleaner' code-wise. Some Ruby source code demonstrating RubyInline is available.
Inspired by the newsgroup thread, Pat Eyler wrote " RubyInline, Making Making Things Faster Easier" with a demonstration and benchmark results of how he used RubyInline to significantly speed up the calculation of prime numbers, and then immediately followed it up with "RubyInline: Going a bit Further". Both articles provide a good look at the performance gains possible by using inline C.