Why ReadAsync is faster in FileStream vs StreamReader in C#? - async-await

Question

I am writing a method to read large files using new ReadAsync methods. In my testing looks like FileStream ReadAsync is faster than StreamReader, not sure why?
ReadStreamReaderAsync
Thread ID Before Await : 9
Thread ID After Await : 13
Time : 76626
Total Bytes : 687184052
ReadFileStreamAsync
Thread ID Before Await : 9
Thread ID After Await : 10
Time : 19167
Total Bytes : 687184052
static async Task<long> ReadStreamReaderAsync(string filename)
{
Console.WriteLine("Thread ID Before Await : {0}", System.Threading.Thread.CurrentThread.ManagedThreadId);
long totalBytes = 0;
var sp = new Stopwatch();
sp.Start();
using (StreamReader reader = new StreamReader(filename, Encoding.Default))
{
char[] buffer = new char[0x1000];
int numRead;
while ((numRead = await reader.ReadAsync(buffer, 0, buffer.Length)) != 0)
{
totalBytes += numRead;
}
}
sp.Stop();
Console.WriteLine("Thread ID After Await : {0}", System.Threading.Thread.CurrentThread.ManagedThreadId);
Console.WriteLine("Time : {0}", sp.ElapsedMilliseconds);
return totalBytes;
}
static async Task<long> ReadFileStreamAsync(string filePath)
{
Console.WriteLine("Thread ID Before Await : {0}", System.Threading.Thread.CurrentThread.ManagedThreadId);
long totalBytes = 0;
var sp = new Stopwatch();
sp.Start();
using (FileStream sourceStream = new FileStream(filePath,
FileMode.Open, FileAccess.Read, FileShare.Read,
bufferSize: 4096, useAsync: true))
{
byte[] buffer = new byte[0x1000];
int numRead;
while ((numRead = await sourceStream.ReadAsync(buffer, 0, buffer.Length)) != 0)
{
totalBytes += numRead;
}
}
sp.Stop();
Console.WriteLine("Thread ID After Await : {0}", System.Threading.Thread.CurrentThread.ManagedThreadId);
Console.WriteLine("Time : {0}", sp.ElapsedMilliseconds);
return totalBytes;
}

Answer 1

StreamReader is using an encoder to read the data (UTF-8).
If you have used a UTF-8 based file you will potentially have received less data as UTF-8 can encoded to many bytes and the Encoder will understand this.
FileStream is dumb and is giving you raw data and trusting you know how to handle it. So if for instance you are reading a text file you should use StreamReader (with the correct encoder)