diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index 7146a97195eb..66fe8b4ebd57 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -77,6 +77,9 @@ Optimizations
 
 * GITHUB#13961: Replace Map<String,Object> with IntObjectHashMap for DV producer. (Pan Guixin)
 
+* GITHUB#13963: Speed up nextDoc() implementations in Lucene912PostingsReader.
+  (Adrien Grand)
+
 Bug Fixes
 ---------------------
 * GITHUB#13832: Fixed an issue where the DefaultPassageFormatter.format method did not format passages as intended
diff --git a/lucene/core/src/java/org/apache/lucene/codecs/lucene912/Lucene912PostingsReader.java b/lucene/core/src/java/org/apache/lucene/codecs/lucene912/Lucene912PostingsReader.java
index cea329c93ca3..6da66e4ddf2a 100644
--- a/lucene/core/src/java/org/apache/lucene/codecs/lucene912/Lucene912PostingsReader.java
+++ b/lucene/core/src/java/org/apache/lucene/codecs/lucene912/Lucene912PostingsReader.java
@@ -580,7 +580,7 @@ private void moveToNextLevel0Block() throws IOException {
 
     @Override
     public int nextDoc() throws IOException {
-      if (doc == level0LastDocID) { // advance skip data on level 0
+      if (docBufferUpto == BLOCK_SIZE) { // advance skip data on level 0
         moveToNextLevel0Block();
       }
 
@@ -875,7 +875,7 @@ private void moveToNextLevel0Block() throws IOException {
 
     @Override
     public int nextDoc() throws IOException {
-      if (doc == level0LastDocID) { // advance level 0 skip data
+      if (docBufferUpto == BLOCK_SIZE) { // advance level 0 skip data
         moveToNextLevel0Block();
       }
 
@@ -1417,11 +1417,13 @@ private void moveToNextLevel0Block() throws IOException {
 
     @Override
     public int nextDoc() throws IOException {
-      if (doc == level0LastDocID) {
-        moveToNextLevel0Block();
-      } else if (needsRefilling) {
-        refillDocs();
-        needsRefilling = false;
+      if (docBufferUpto == BLOCK_SIZE) {
+        if (needsRefilling) {
+          refillDocs();
+          needsRefilling = false;
+        } else {
+          moveToNextLevel0Block();
+        }
       }
 
       return this.doc = (int) docBuffer[docBufferUpto++];
@@ -1644,8 +1646,9 @@ public void advanceShallow(int target) throws IOException {
 
     @Override
     public int nextDoc() throws IOException {
-      advanceShallow(doc + 1);
-      if (needsRefilling) {
+      if (docBufferUpto == BLOCK_SIZE) {
+        advanceShallow(doc + 1);
+        assert needsRefilling;
         refillDocs();
         needsRefilling = false;
       }