don't use .last(3), use .limit(3) instead. also, don't put an order on an association as it isn't overridable. (this commit minimizes AR object instantiation in the stream)

app/models/post.rb

@@ -67,7 +67,9 @@ class Post < ActiveRecord::Base
   # gives the last three comments on the post
   def last_three_comments
     return if self.comments_count == 0
-    self.comments.includes(:author => :profile).last(3)
+    # DO NOT USE .last(3) HERE.  IT WILL FETCH ALL COMMENTS AND RETURN THE LAST THREE
+    # INSTEAD OF DOING THE FOLLOWING, AS EXPECTED (THX AR):
+    self.comments.order('created_at DESC').limit(3).includes(:author => :profile).reverse!
   end
 
   def self.excluding_blocks(user)

lib/diaspora/commentable.rb

@@ -6,7 +6,7 @@ module Diaspora
   module Commentable
     def self.included(model)
       model.instance_eval do
-        has_many :comments, :as => :commentable, :order => 'created_at', :dependent => :destroy
+        has_many :comments, :as => :commentable, :dependent => :destroy
       end
     end