plataformatec/devise — pull request #2913 by petervandenabeele (2014-03-02 17:58:17+00:00)
Title: Note that the cost of bcrypt grows dramatically with number of stretches

* this will avoid people (like me) loosing an hour trying to understand
why doing it "a little bit safer" with 20 stretches suddenly takes
60 seconds to do sign_up or sign_in. An example of such discussion is:
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/399627