While the Ruby 1.8 VM is quite slow, the YARV VM used in 1.9 is much faster, resulting in similar performance to most other dynamic languages. These tests are showing MacRuby being 3x faster than 1.9, which puts it in "quite fast" territory.
Recent research has tended to show that the Abominable No-Man is being replaced by the Prohibitive Procrastinator. -- C.N. Parkinson