I don't think so. Java outperforms JRuby, probably because of typing. See: http://shootout.alioth.debian.org/u64q/benchmark.php?test=al...
But in any case, if Twitter's architecture is truly scalable then any intrinsic slowness of the language shouldn't be a big problem, because they can just toss more hardware in to compensate. What is a problem is a buggy VM that leaks memory. To run thousands of instances in a heavily instrumented way, the VM must be stable and predictable.
JavaScript
http://shootout.alioth.debian.org/u64/which-programs-are-fas...