The problem with performance was in handling the data in the loop. This code has managed to reduce processing from over 2 minutes to just 2 seconds.
string _path = @"C:\Output.txt";
string _text = string.Empty;
for (int x = 0; x < 10000; x++)
{
_text = _text + "This is line " + x.ToString() + Environment.NewLine;
}
_AppendDataFile(_path, _text);