c#数据转换太慢。。。

string path = @"frame_0_right.dat";
byte[] data = File.ReadAllBytes(path); //按字节读取
string returnStr = "";
int Length = data.Length;
Console.Write(Length);
Console.WriteLine("/n"); //正常Length可以立马输出
for (int i = 0; i < Length; i++)
{
returnStr += data[i].ToString("X2"); /*returnStr则要等好久，如果数据 千万级，根本出不来*/
}Console.Write(returnStr);
求问怎么办!!

StringBuilder sb = new StringBuilder();
for (int i = 0; i < Length; i++)
{
sb.Append(data[i].ToString("X2"));
}
returnStr = sb.ToString();