diff --git a/activesupport/lib/active_support/core_ext/class/attribute.rb b/activesupport/lib/active_support/core_ext/class/attribute.rb
index 1c3d26ead4f72..1504e18839d3f 100644
--- a/activesupport/lib/active_support/core_ext/class/attribute.rb
+++ b/activesupport/lib/active_support/core_ext/class/attribute.rb
@@ -72,6 +72,9 @@ def class_attribute(*attrs)
     instance_reader = options.fetch(:instance_accessor, true) && options.fetch(:instance_reader, true)
     instance_writer = options.fetch(:instance_accessor, true) && options.fetch(:instance_writer, true)
 
+    # We use class_eval here rather than define_method because class_attribute
+    # may be used in a performance sensitive context therefore the overhead that
+    # define_method introduces may become significant.
     attrs.each do |name|
       class_eval <<-RUBY, __FILE__, __LINE__ + 1
         def self.#{name}() nil end