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 083b4ee1be..46bd380948 100644
--- a/codec/src/main/java/io/netty/handler/codec/ByteToMessageDecoder.java
+++ b/codec/src/main/java/io/netty/handler/codec/ByteToMessageDecoder.java
@@ -80,6 +80,11 @@ public abstract class ByteToMessageDecoder extends ChannelInboundHandlerAdapter
     public static final Cumulator MERGE_CUMULATOR = new Cumulator() {
         @Override
         public ByteBuf cumulate(ByteBufAllocator alloc, ByteBuf cumulation, ByteBuf 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() ||
@@ -91,7 +96,9 @@ public abstract class ByteToMessageDecoder extends ChannelInboundHandlerAdapter
                     //   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)
@@ -108,31 +115,33 @@ public abstract class ByteToMessageDecoder extends ChannelInboundHandlerAdapter
     public static final Cumulator COMPOSITE_CUMULATOR = new Cumulator() {
         @Override
         public ByteBuf cumulate(ByteBufAllocator alloc, ByteBuf cumulation, ByteBuf 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();
+                    }
                 }
             }
         }
@@ -261,13 +270,9 @@ public abstract class ByteToMessageDecoder extends ChannelInboundHandlerAdapter
         if (msg instanceof ByteBuf) {
             CodecOutputList out = CodecOutputList.newInstance();
             try {
-                ByteBuf data = (ByteBuf) msg;
                 first = cumulation == null;
-                if (first) {
-                    cumulation = data;
-                } else {
-                    cumulation = cumulator.cumulate(ctx.alloc(), cumulation, data);
-                }
+                cumulation = cumulator.cumulate(ctx.alloc(),
+                        first ? Unpooled.EMPTY_BUFFER : cumulation, (ByteBuf) msg);
                 callDecode(ctx, cumulation, out);
             } catch (DecoderException e) {
                 throw e;
@@ -517,13 +522,18 @@ public abstract class ByteToMessageDecoder extends ChannelInboundHandlerAdapter
         }
     }
 
-    private static ByteBuf expandCumulation(ByteBufAllocator alloc, ByteBuf oldCumulation, ByteBuf in) {
-        ByteBuf newCumulation = alloc.buffer(alloc.calculateNewCapacity(
-                oldCumulation.readableBytes() + in.readableBytes(), MAX_VALUE));
+    static ByteBuf expandCumulation(ByteBufAllocator alloc, ByteBuf oldCumulation, ByteBuf in) {
+        int oldBytes = oldCumulation.readableBytes();
+        int newBytes = in.readableBytes();
+        int totalBytes = oldBytes + newBytes;
+        ByteBuf newCumulation = alloc.buffer(alloc.calculateNewCapacity(totalBytes, MAX_VALUE));
         ByteBuf toRelease = newCumulation;
         try {
-            newCumulation.writeBytes(oldCumulation);
-            newCumulation.writeBytes(in);
+            // This avoids redundant checks and stack depth compared to calling writeBytes(...)
+            newCumulation.setBytes(0, oldCumulation, oldCumulation.readerIndex(), oldBytes)
+                .setBytes(oldBytes, in, in.readerIndex(), newBytes)
+                .writerIndex(totalBytes);
+            in.readerIndex(in.writerIndex());
             toRelease = oldCumulation;
             return newCumulation;
         } finally {
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 4279ece838..3db8022c41 100644
--- a/codec/src/test/java/io/netty/handler/codec/ByteToMessageDecoderTest.java
+++ b/codec/src/test/java/io/netty/handler/codec/ByteToMessageDecoderTest.java
@@ -325,11 +325,11 @@ 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() {
@@ -340,6 +340,7 @@ 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;
@@ -363,7 +364,7 @@ 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) {
@@ -415,7 +416,11 @@ 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);