The second option is the best one as ToArray will in fact create a copy of the internal array stored in the MemoryStream.

But, you can also preferably use memoryStr.GetBuffer() that will return a reference to this internal array. In this case, you need to use the memoryStr.Length property because the buffer returned by GetBuffer() is in general bigger than the stored actual data (it's allocated chunk by chunk, not byte by byte).

Note that it would be best to pass the original data as a stream directly to the ASP.NET outputstream, instead of using an intermediary MemoryStream. It depends on how you get your binary data in the first place.

Another option, if you serve the exact same content often, is to save this MemoryStream to a physical file (using a FileStream), and use Response.TransmitFile on all subsequent requests. Response.TransmitFile is using low level Windows socket layers and there's nothing faster to send a file.