Ruby is great at many things, and it has adequate performance for many situations -'Fast Enough', as DHH said. And with liberal use of C extensions, I've used it in some high-performance scenarios where the expressiveness was more beneficial than the complexity of using extensions.
But it's really not fast – up to 100x slower than Java (http://benchmarksgame.alioth.debian.org/u32/compare.php?lang...) or 30x slower than Javascript on V8 (http://benchmarksgame.alioth.debian.org/u32/compare.php?lang...). I know these are micro benchmarks, but they're useful for comparison.
Some of this is down to the nature of the Ruby interpreter and will improve with time (and with implementations like JRuby). Some of it is down to the highly dynamic nature of the language. The point remains though – if you need high performance for a particular task, Ruby may not be the best option.