Hi,
I can't remember whether I promised more performance patches, but anyways here
they are 
http://dev.rubyonrails.org/ticket/11108 By rewriting string callbacks to symbol callbacks in AR associations we can get significant performance improvement because there's no need to create extra Binding object for each string callback to be evaluated with eval(). The performance improvement is tangible even for individual model operations, not saying anything about loops (I saved 50M memory and 700ms because of this change).
http://dev.rubyonrails.org/ticket/11109 This change is quite controversial, but it also has effect. method_missing is often called for AR associations to pass method calls to underlying array but method missing has &block parameter and thus requires Ruby to create Proc object and Binding along with it. This allocates noticeable amount of memory and can be avoided replacing each call like     def foo(&block)         bar(&block)     end with     def foo         bar { |*block_args| yield(*block_args) if block_given?     end
I've explained the problem and performance gains in more details in ticket descriptions and also in the blog: http://blog.pluron.com/2008/02/rails-faster-as.html
http://dev.rubyonrails.org/ticket/11110 The idea of this small performance improvement is that most likely people will not subclass from BigDecimal's and Rails' type_cast method will always get BigDecimal's. Therefore we can save a couple of milliseconds when checking if the value is already BigDecimal using class equality instead of #is_a? method.
http://dev.rubyonrails.org/ticket/11111 This is the simplest change. AR::Base#attributes_with_quotes likes to call AR::Base#connection methods. By cashing the return value of the #connection method we can save a couple of milliseconds.