improve performance by processing a buffer, not a string

[shaunlebron]

from: http://ergoemacs.org/emacs/elisp_read_file_content.html

Note: in elisp, it's more efficient to process text in a buffer than doing complicated string manipulation with string data type. But, if your lines are all short and you don't need to know the text that comes before or after current line, then, list of lines can be easier to work with.