diff --git a/activesupport/lib/active_support/notifications/instrumenter.rb b/activesupport/lib/active_support/notifications/instrumenter.rb
index <HASH>..<HASH> 100644
--- a/activesupport/lib/active_support/notifications/instrumenter.rb
+++ b/activesupport/lib/active_support/notifications/instrumenter.rb
@@ -68,7 +68,6 @@ module ActiveSupport
         @transaction_id = transaction_id
         @end            = ending
         @children       = []
-        @duration       = nil
         @cpu_time_start = 0
         @cpu_time_finish = 0
         @allocation_count_start = 0
@@ -125,7 +124,7 @@ module ActiveSupport
       #
       #   @event.duration # => 1000.138
       def duration
-        @duration ||= 1000.0 * (self.end - time)
+        1000.0 * (self.end - time)
       end
 
       def <<(event)
Remove @duration instance variable since we're not maintaining instances variables such as @cpu_time, @idle_time and @allocations; this reduces one allocation