Download speed radically change depending on function used

Hello,

We have a problem with the speed of our downloads. And depending on method, it can go 20x faster/slower.

The slow method is HttpContent.ReadAsStreamAsync(). We use this method so we can make a progression on the download.
I tried to use HttpContent.ReadAsByteArrayAsync(), this is so much faster !!

Is it normal ? How can I improve download speed of ReadAsStreamAsync ?
It tried to enlarge the size of the buffer, that didnt change anything...

Weird thing : ReadAsStreamAsync speed seems relatively OK when running on an emulator.

Simplified code below for the 2 tests

Slow method :

int bufferSizeHttp = 1024 * 4;
    using (Stream stream = await response.Content.ReadAsStreamAsync())
                {                
                    do
                    {                    
                        var percentBefore = percent;                    
                        try
                        {   
                            var bytesRead = await stream.ReadAsync(buffer, 0, bufferSizeHttp);

                            if (bytesRead > 0)
                            {
                                var data = new byte[bytesRead];
                                buffer.ToList().CopyTo(0, data, 0, bytesRead);
                                await saveFileStream.WriteAsync(data, 0, data.Length);
                                await saveFileStream.FlushAsync();

                                totalBytesRead += bytesRead;
                                percent = Math.Floor((double)(totalBytesRead * 100 / totalBytes));
                            }

                //other stuff

Fast method :

var data = await response.Content.ReadAsByteArrayAsync();
                do
                {                    
                    var percentBefore = percent;                    
                    try
                    {
                        var bytesRead = data.Length;

                        if (bytesRead > 0)
                        {
                            await saveFileStream.WriteAsync(data, 0, bytesRead);
                            await saveFileStream.FlushAsync();

                            totalBytesRead += bytesRead;
                            percent = Math.Floor((double)(totalBytesRead * 100 / totalBytes));
                        }

            //other stuff