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This begins to look good.

What's a normal text file?  One with a million bytes? :-)

Have you made sure this works as expected in Universal
newline mode?

I'd like a patch that doesn't use #define WITH_READAHEAD_BUFFER.

You might also experiment with larger buffer sizes (I
predict that a larger buffer doesn't make much difference,
since it didn't for xreadlines, but it would be nice to
verify that and then add a comment; at least once a year
someone asks whether the buffer shouldn't be much larger).