We use Rails 3 with a boat load of gems, so the basic boot time for any Rake task or Rails Generator is slow... about 1 minute.
Turns out, the basic performance parameters are far from ideal for a big Rails app. As stated at Reddit, there are some tuning parameters. Put this in your bashrc/zshrc and/or
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We decreased our start-up time by 80%!
Specs: Ruby 1.9.3p194 with Rails 3.2.6 with about 100 gems.