diff --git a/app/assets/javascripts/discourse/models/composer.js.es6 b/app/assets/javascripts/discourse/models/composer.js.es6
index 049bba2eb4..d0949175a2 100644
--- a/app/assets/javascripts/discourse/models/composer.js.es6
+++ b/app/assets/javascripts/discourse/models/composer.js.es6
@@ -56,7 +56,8 @@ const CLOSED = "closed",
     categoryId: "topic.category.id",
     tags: "topic.tags",
     featuredLink: "topic.featured_link"
-  };
+  },
+  FAST_REPLY_LENGTH_THRESHOLD = 10000;
 
 export const SAVE_LABELS = {
   [EDIT]: "composer.save_edit",
@@ -445,10 +446,62 @@ const Composer = RestModel.extend({
   @computed("reply")
   replyLength(reply) {
     reply = reply || "";
-    while (Quote.REGEXP.test(reply)) {
-      reply = reply.replace(Quote.REGEXP, "");
+
+    if (reply.length > FAST_REPLY_LENGTH_THRESHOLD) {
+      return reply.length;
     }
-    return reply.replace(/\s+/gim, " ").trim().length;
+
+    if (Quote.REGEXP.test(reply)) {
+      // make it global so we can strip all quotes at once
+      const regex = new RegExp(Quote.REGEXP.source, "img");
+      reply = reply.replace(regex, "");
+    }
+
+    // This is in place so we do not generate any intermediate
+    // strings while calculating the length, this is issued
+    // every keypress in the composer so it needs to be very fast
+    let len = 0,
+      skipSpace = true;
+
+    for (let i = 0; i < reply.length; i++) {
+      const code = reply.charCodeAt(i);
+
+      let isSpace = false;
+      if (code >= 0x2000 && code <= 0x200a) {
+        isSpace = true;
+      } else {
+        switch (code) {
+          case 0x09: // \t
+          case 0x0a: // \n
+          case 0x0b: // \v
+          case 0x0c: // \f
+          case 0x0d: // \r
+          case 0x20:
+          case 0xa0:
+          case 0x1680:
+          case 0x202f:
+          case 0x205f:
+          case 0x3000:
+            isSpace = true;
+        }
+      }
+
+      if (isSpace) {
+        if (!skipSpace) {
+          len++;
+          skipSpace = true;
+        }
+      } else {
+        len++;
+        skipSpace = false;
+      }
+    }
+
+    if (len > 0 && skipSpace) {
+      len--;
+    }
+
+    return len;
   },
 
   @on("init")