An all-ASCII fast-path is basically obligatory for anything that cares about performance. And then the "slow path" gets a guarantee that there will be varying-l

An all-ASCII fast-path is basically obligatory for anything that cares about performance. And then the "slow path" gets a guarantee that there will be varying-length data, and, as you'll want it to not actually be slow, you'll still need to avoid branching on the now-guaranteed-variable width.