This method is always used on sliced ByteBuf (as the first 4 bytes old the
overall size) so the payload will never match the underlying buffer's array
thus defeats the purpose of the optimization and end up costing CPU cycles.

On a typical application this represent ~0.4% of the CPU

Directly read the source ByteBuf while checking for
content length. Reader index are similarily moved.

On a typical application this takes ~0.88% of CPU