diff --git a/codec/src/main/java/io/netty/handler/codec/ByteToMessageDecoder.java b/codec/src/main/java/io/netty/handler/codec/ByteToMessageDecoder.java
index a51ab57105..1c5df26c31 100644
--- a/codec/src/main/java/io/netty/handler/codec/ByteToMessageDecoder.java
+++ b/codec/src/main/java/io/netty/handler/codec/ByteToMessageDecoder.java
@@ -90,6 +90,11 @@ public abstract class ByteToMessageDecoder extends ChannelHandlerAdapter {
      * Cumulate {@link ByteBuf}s by merge them into one {@link ByteBuf}'s, using memory copies.
      */
     public static final Cumulator MERGE_CUMULATOR = (alloc, cumulation, in) -> {
+        if (!cumulation.isReadable() && in.isContiguous()) {
+            // If cumulation is empty and input buffer is contiguous, use it directly
+            cumulation.release();
+            return in;
+        }
         try {
             final int required = in.readableBytes();
             if (required > cumulation.maxWritableBytes() ||
@@ -101,7 +106,9 @@ public abstract class ByteToMessageDecoder extends ChannelHandlerAdapter {
                 //   assumed to be shared (e.g. refCnt() > 1) and the reallocation may not be safe.
                 return expandCumulation(alloc, cumulation, in);
             }
-            return cumulation.writeBytes(in);
+            cumulation.writeBytes(in, in.readerIndex(), required);
+            in.readerIndex(in.writerIndex());
+            return cumulation;
         } finally {
             // We must release in in all cases as otherwise it may produce a leak if writeBytes(...) throw
             // for whatever release (for example because of OutOfMemoryError)
@@ -116,31 +123,33 @@ public abstract class ByteToMessageDecoder extends ChannelHandlerAdapter {
      */
 
     public static final Cumulator COMPOSITE_CUMULATOR = (alloc, cumulation, in) -> {
+        if (!cumulation.isReadable()) {
+            cumulation.release();
+            return in;
+        }
+        CompositeByteBuf composite = null;
         try {
-            if (cumulation.refCnt() > 1) {
-                // Expand cumulation (by replace it) when the refCnt is greater then 1 which may happen when the
-                // user use slice().retain() or duplicate().retain().
-                //
-                // See:
-                // - https://github.com/netty/netty/issues/2327
-                // - https://github.com/netty/netty/issues/1764
-                return expandCumulation(alloc, cumulation, in);
-            }
-            final CompositeByteBuf composite;
-            if (cumulation instanceof CompositeByteBuf) {
+            if (cumulation instanceof CompositeByteBuf && cumulation.refCnt() == 1) {
                 composite = (CompositeByteBuf) cumulation;
+                // Writer index must equal capacity if we are going to "write"
+                // new components to the end
+                if (composite.writerIndex() != composite.capacity()) {
+                    composite.capacity(composite.writerIndex());
+                }
             } else {
-                composite = alloc.compositeBuffer(MAX_VALUE);
-                composite.addComponent(true, cumulation);
+                composite = alloc.compositeBuffer(Integer.MAX_VALUE).addFlattenedComponents(true, cumulation);
             }
-            composite.addComponent(true, in);
+            composite.addFlattenedComponents(true, in);
             in = null;
             return composite;
         } finally {
             if (in != null) {
-                // We must release if the ownership was not transferred as otherwise it may produce a leak if
-                // writeBytes(...) throw for whatever release (for example because of OutOfMemoryError).
+                // We must release if the ownership was not transferred as otherwise it may produce a leak
                 in.release();
+                // Also release any new buffer allocated if we're not returning it
+                if (composite != null && composite != cumulation) {
+                    composite.release();
+                }
             }
         }
     };
diff --git a/codec/src/test/java/io/netty/handler/codec/ByteToMessageDecoderTest.java b/codec/src/test/java/io/netty/handler/codec/ByteToMessageDecoderTest.java
index eda48af9a8..9822d31d26 100644
--- a/codec/src/test/java/io/netty/handler/codec/ByteToMessageDecoderTest.java
+++ b/codec/src/test/java/io/netty/handler/codec/ByteToMessageDecoderTest.java
@@ -324,11 +324,11 @@ public class ByteToMessageDecoderTest {
         }
 
         @Override
-        public ByteBuf writeBytes(ByteBuf src) {
+        public ByteBuf setBytes(int index, ByteBuf src, int srcIndex, int length) {
             if (--untilFailure <= 0) {
                 throw error;
             }
-            return super.writeBytes(src);
+            return super.setBytes(index, src, srcIndex, length);
         }
 
         Error writeError() {
@@ -339,6 +339,7 @@ public class ByteToMessageDecoderTest {
     @Test
     public void releaseWhenMergeCumulateThrows() {
         WriteFailingByteBuf oldCumulation = new WriteFailingByteBuf(1, 64);
+        oldCumulation.writeZero(1);
         ByteBuf in = Unpooled.buffer().writeZero(12);
 
         Throwable thrown = null;
@@ -362,7 +363,7 @@ public class ByteToMessageDecoderTest {
     }
 
     private void releaseWhenMergeCumulateThrowsInExpand(int untilFailure, boolean shouldFail) {
-        ByteBuf oldCumulation = UnpooledByteBufAllocator.DEFAULT.heapBuffer(8, 8);
+        ByteBuf oldCumulation = UnpooledByteBufAllocator.DEFAULT.heapBuffer(8, 8).writeZero(1);
         final WriteFailingByteBuf newCumulation = new WriteFailingByteBuf(untilFailure, 16);
 
         ByteBufAllocator allocator = new AbstractByteBufAllocator(false) {
@@ -414,7 +415,11 @@ public class ByteToMessageDecoderTest {
             public CompositeByteBuf addComponent(boolean increaseWriterIndex, ByteBuf buffer) {
                 throw error;
             }
-        };
+            @Override
+            public CompositeByteBuf addFlattenedComponents(boolean increaseWriterIndex, ByteBuf buffer) {
+                throw error;
+            }
+        }.writeZero(1);
         ByteBuf in = Unpooled.buffer().writeZero(12);
         try {
             ByteToMessageDecoder.COMPOSITE_CUMULATOR.cumulate(UnpooledByteBufAllocator.DEFAULT, cumulation, in);