>(1) You are constructing a StringBuilder each time you want a line. Can you work on the string directly ?
>(2) I suspect the At() takes a long time - especially if you want the 900th line or so
>(3) Instead of reading the directory into one string, can you read it into an array or strings ? That way the access to any line would be fast and you would not have the overhead of splitting each time you want a specific line

Thanks