On my system it takes about 63/100ths of a second to read line by line through a file of 3.5 million lines that is 272 megabytes in size (that's the closest to 231MB and 2.7M lines that I happened to have laying around). That's with a no-op loop; whatever you do to process the lines of the file will consume time too, but they will consume virtually the same time whether you're iterating over lines from a file, or the elements of an array.

If performance is an issue, profile.