Antonio Cangiano posted a Ruby Implementation Shootout on his blog last week. While it's an interesting piece (and will likely be more interesting over time), it's still very premature.
The various implementations are still don't pass all the tests involved in the shootout (particularily rubinius and Cardinal). In fact, most of them have either not done any optimization work, or are just starting down that road.
Still, YARV shows up quite well in the test as it stands, and JRuby and Ruby.NET both show a lot of promise. It will be interesting to see how the numbers look in 6 months or so.
This doesn't mean that no one is looking at Ruby 1.8.5 speed though. Tomasz Wegrzanowski unveiled a post called Making Ruby Faster, in which he shows some opportunities for speeding up the stock interpreter. Hopefully he (and others on the ruby-core mailing list) will be able to get some improvements into 1.8.6.