Reading from file not fast enough, how would I speed it up?

public static string readFile(string path)
    {
        StringBuilder stringFromFile = new StringBuilder();
        StreamReader SR;
        string S;
        SR = File.OpenText(path);
        S = SR.ReadLine();
        while (S != null)
        {
            stringFromFile.Append(SR.ReadLine());
        }
        SR.Close();
        return stringFromFile.ToString();
    }

public static string readFile(string path)
    {

       return File.ReadAllText(path);

    }

S = SR.ReadLine();
while (S != null)
{
    stringFromFile.Append(SR.ReadLine());
}

S = SR.ReadLine();
while (S != null)
{
    stringFromFile.Append(S = SR.ReadLine());
}

S = SR.ReadLine();
while (S != null){
    stringFromFile.Append(S);
    S = SR.ReadLine();
}

return System.IO.File.ReadAllText(path);

var str = System.IO.File.ReadAllText(path);
return str.Replace(Environment.NewLine, "");

var info = new FileInfo(path);
var sb = new StringBuilder((int)info.Length);