diff --git a/lib/fluent/plugin/buffer.rb b/lib/fluent/plugin/buffer.rb
index 7d07307dab..02c702affd 100644
--- a/lib/fluent/plugin/buffer.rb
+++ b/lib/fluent/plugin/buffer.rb
@@ -192,6 +192,7 @@ def initialize
 
         @stage_size = @queue_size = 0
         @timekeys = Hash.new(0)
+        @enable_update_timekeys = false
         @mutex = Mutex.new
       end
 
@@ -207,6 +208,10 @@ def configure(conf)
         end
       end
 
+      def enable_update_timekeys
+        @enable_update_timekeys = true
+      end
+
       def start
         super
 
@@ -268,8 +273,9 @@ def new_metadata(timekey: nil, tag: nil, variables: nil)
         Metadata.new(timekey, tag, variables)
       end
 
+      # Keep this method for existing code
       def metadata(timekey: nil, tag: nil, variables: nil)
-        meta = Metadata.new(timekey, tag, variables)
+        Metadata.new(timekey, tag, variables)
       end
 
       def timekeys
@@ -465,8 +471,10 @@ def enqueue_unstaged_chunk(chunk)
 
       def update_timekeys
         synchronize do
-          @timekeys = (@stage.values + @queue).each_with_object({}) do |chunk, keys|
-            if chunk && chunk.metadata && chunk.metadata.timekey
+          chunks = @stage.values
+          chunks.concat(@queue)
+          @timekeys = chunks.each_with_object({}) do |chunk, keys|
+            if chunk.metadata && chunk.metadata.timekey
               t = chunk.metadata.timekey
               keys[t] = keys.fetch(t, 0) + 1
             end
@@ -477,7 +485,7 @@ def update_timekeys
       # At flush_at_shutdown, all staged chunks should be enqueued for buffer flush. Set true to force_enqueue for it.
       def enqueue_all(force_enqueue = false)
         log.on_trace { log.trace "enqueueing all chunks in buffer", instance: self.object_id }
-        update_timekeys
+        update_timekeys if @enable_update_timekeys
 
         if block_given?
           synchronize{ @stage.keys }.each do |metadata|
@@ -787,11 +795,11 @@ def statistics
           'total_queued_size' => stage_size + queue_size,
         }
 
-        if (m = timekeys.min)
+        tkeys = timekeys
+        if (m = tkeys.min)
           stats['oldest_timekey'] = m
         end
-
-        if (m = timekeys.max)
+        if (m = tkeys.max)
           stats['newest_timekey'] = m
         end
 
diff --git a/lib/fluent/plugin/output.rb b/lib/fluent/plugin/output.rb
index b5128bc8e2..8e9950fd5b 100644
--- a/lib/fluent/plugin/output.rb
+++ b/lib/fluent/plugin/output.rb
@@ -340,6 +340,7 @@ def configure(conf)
           buffer_conf = conf.elements(name: 'buffer').first || Fluent::Config::Element.new('buffer', '', {}, [])
           @buffer = Plugin.new_buffer(buffer_type, parent: self)
           @buffer.configure(buffer_conf)
+          @buffer.enable_update_timekeys if @chunk_key_time
 
           @flush_at_shutdown = @buffer_config.flush_at_shutdown
           if @flush_at_shutdown.nil?