Sending data through a TCP stream
If you are looking for ways to optimize your application - you are clearly looking at the wrong place, at least in my opinion. BinaryWriter is a simple wrapper around actual stream. All it does is it converts simple types to byte arrays and writes those to stream. Ofc it is slower, as any other wrapper, but not that much slower. In real life this difference can be ignored.

If writing to NetworkStream is your main performance issue, then consider wrapping it with BufferedStream. 

If it isnt, and you are simply curious... then you can always use disassembler to see whats inside BinaryWriter.Write method for yourself.