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 /etc/profile:

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export RUBY_HEAP_MIN_SLOTS=800000
export RUBY_HEAP_FREE_MIN=100000
export RUBY_HEAP_SLOTS_INCREMENT=300000
export RUBY_HEAP_SLOTS_GROWTH_FACTOR=1
export RUBY_GC_MALLOC_LIMIT=79000000

Result

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Before: rake -T  23,05s user 1,39s system 47% cpu 51,545 total
After:  rake -T  6,79s  user 0,98s system 85% cpu 9,124 total

We decreased our start-up time by 80%!

Specs: Ruby 1.9.3p194 with Rails 3.2.6 with about 100 gems.

Thanks outsmartin for finding this piece on Reddit. You are this week's "Hero of Labor" :)