Likely, comparing on `char` ('+') would be slower as it requires decoding the `&str` as a `char` which comes with some significant overhead (I've seen 9% on a fairly optimized parser). Ideally, when you grammar is 7-bit ASCII (or any 8-bit UTF-8 values are opaque to your grammar), you instead parse on `&[u8]` and do `u8` comparisons, rather than `char` or `&[u8]`.
