diff --git a/src/General/Index.coffee b/src/General/Index.coffee
index 921d31c1d..c67022a5c 100644
--- a/src/General/Index.coffee
+++ b/src/General/Index.coffee
@@ -687,10 +687,6 @@ Index =
   isHiddenReply: (threadID, replyData) ->
     PostHiding.isHidden(g.BOARD.ID, threadID, replyData.no) or Filter.isHidden(g.SITE.Build.parseJSON replyData, g.BOARD.ID)
 
-  lastPost: (threadID) ->
-    threadData = Index.liveThreadDict[threadID]
-    if threadData?.last_replies then threadData.last_replies[threadData.last_replies.length - 1].no else threadID
-
   buildThreads: (threadIDs, isCatalog, withReplies) ->
     threads    = []
     newThreads = []
@@ -712,6 +708,8 @@ Index =
         else
           thread = new Thread ID, g.BOARD
           newThreads.push thread
+        lastPost = if threadData.last_replies then threadData.last_replies[threadData.last_replies.length - 1].no else ID
+        thread.lastPost = lastPost if lastPost > thread.lastPost
         thread.json = threadData
         threads.push thread
 
diff --git a/src/Monitoring/UnreadIndex.coffee b/src/Monitoring/UnreadIndex.coffee
index 1ca6c489c..f7d8afc04 100644
--- a/src/Monitoring/UnreadIndex.coffee
+++ b/src/Monitoring/UnreadIndex.coffee
@@ -75,7 +75,7 @@ UnreadIndex =
     hasUnread = if repliesShown
       firstUnread or !repliesRead
     else if Index.enabled
-      Index.lastPost(thread.ID) > lastReadPost
+      thread.lastPost > lastReadPost
     else
       thread.OP.ID > lastReadPost
     thread.nodes.root.classList.toggle 'unread-thread', hasUnread
@@ -94,15 +94,11 @@ UnreadIndex =
 
   markRead: ->
     thread = Get.threadFromNode @
-    lastPost = if Index.enabled then Index.lastPost(thread.ID) else 0
-    thread.posts.forEach (post) ->
-      if post.ID > lastPost and !post.isFetchedQuote
-        lastPost = post.ID
-    UnreadIndex.lastReadPost[thread.fullID] = lastPost
+    UnreadIndex.lastReadPost[thread.fullID] = thread.lastPost
     UnreadIndex.db.set
       boardID:  thread.board.ID
       threadID: thread.ID
-      val:      lastPost
+      val:      thread.lastPost
     $.rm UnreadIndex.hr[thread.fullID]
     thread.nodes.root.classList.remove 'unread-thread'
     ThreadWatcher.update g.SITE.ID, thread.board.ID, thread.ID,
diff --git a/src/classes/Post.coffee b/src/classes/Post.coffee
index c76a2500b..dfe523db7 100644
--- a/src/classes/Post.coffee
+++ b/src/classes/Post.coffee
@@ -63,6 +63,7 @@ class Post
       @clones = g.posts[@fullID].clones
       clone.origin = @ for clone in @clones
 
+    @thread.lastPost = @ID if !@isFetchedQuote and @ID > @thread.lastPost
     @board.posts.push  @ID, @
     @thread.posts.push @ID, @
     g.posts.push   @fullID, @
diff --git a/src/classes/Thread.coffee b/src/classes/Thread.coffee
index 25f97d0dc..6bfbbd039 100644
--- a/src/classes/Thread.coffee
+++ b/src/classes/Thread.coffee
@@ -11,6 +11,7 @@ class Thread
     @isArchived = false
     @postLimit  = false
     @fileLimit  = false
+    @lastPost   = 0
     @ipCount    = undefined
     @json       = null