i have been practicing test first development (from TDD) approach for last 1.5 years. i wrote a lot of junit test cases and wrote a lot of jmock based mock objects. it was amazing work with it these tools. specially jmock was fantastics and my most favorite mocking tool.
anyway, while i seriously started ruby on rails it is about 2/3 months ago, when i formed my own rails team and started working on first commercial rails project.
at the beginning i was working on IntelliJ IDEA 6M2 on windows environment, where i found running runit test cases are taking longer than we suppose to expect. it was really annoying.
good luck i got my mac book pro where i set up all rails stuff with IntelliJ IDEA 6M2. now i could see a lot more significant difference. now i am really feeling my test environment for rails project over this environment is running more than i expected. it really leads me to belief test driven approach over rails environment is really fast.
it must be some platform related significant changes, which only the expert on that platform can assure me.
by the way, i am really loving intelliJ IDEA and rails over my mac environment.