After reading a number of posts on speeding up tests with Rails:
* http://www.skuunk.com/2012/05/speeding-up-rspec.html
* http://www.dixis.com/?p=553
* http://stackoverflow.com/questions/3663075/speeding-up-rspec-tests-in-a-large-rails-application
* http://www.rubyinside.com/careful-cutting-to-get-faster-rspec-runs-with-rails-5207.html
* http://highgroove.com/articles/2012/04/06/sane-rspec-config-for-clean,-and-slightly-faster,-specs.html
* http://blog.plataformatec.com.br/2011/12/three-tips-to-improve-the-performance-of-your-test-suite/
* https://makandracards.com/makandra/950-speed-up-rspec-by-deferring-garbage-collection
* http://kpumuk.info/ruby-on-rails/my-top-7-rspec-best-practices/

We implemented a combination of practices to reduce our test execution time from 15 minutes for 549 tests down to 7 minutes!:

* Implement the shared connection code to ensure Capybara and non-request tests share a database connection via spec/spec_helper.rb: http://kpumuk.info/ruby-on-rails/my-top-7-rspec-best-practices/

class ActiveRecord::Base
  mattr_accessor :shared_connection
  @@shared_connection = nil
  def self.connection
    @@shared_connection || retrieve_connection
  end
end

* Decrease logging from DEBUG to INFO in spec/spec_helper.rb – http://blog.plataformatec.com.br/2011/12/three-tips-to-improve-the-performance-of-your-test-suite/

 Rails.logger.level = 2

* Switch from rspec transactions to database_cleaner transactions in spec/spec_helper.rb – http://highgroove.com/articles/2012/04/06/sane-rspec-config-for-clean,-and-slightly-faster,-specs.htm. This change sped up test runs by over 30%!

  RSpec.configure do |config|

    config.before(:all) {
      DatabaseCleaner.strategy = :transaction
      DatabaseCleaner.clean_with(:truncation)
    }

    config.before(:each) {
      Rails.application.routes.default_url_options[:host] = %{example.com:80}
      DatabaseCleaner.start
    }

    config.after(:each) {
      DatabaseCleaner.clean
    }

Tried tweaking Ruby’s GC performance with 1.9.2 but couldn’t find settings that helped performance noticeably without starving the system of memory.