> what can be fast

I think this doesn't get talked about enough. If your input is a big run of data that is being checked/transformed in one shot, it works well. But, if you're likely to have to make a decision on several bytes of the input, SIMD will be the same or slower than the scalar method. It's not a magic "go fast" button.

I think the big miss is that the former is achievable far more often than people imagine, which leads to settling for the later, aka pessimization. Reducing your allocations from millions per run to handfuls per run during initialization is effectively a "go fast" button, and is generally more reliable. It is very common for teams to spend big effort getting a 2-3x speedup on allocation-heavy code by disabling branches when a 100-1000x speedup can be had if restructuring allocations is a strategy under consideration (even before the extra 4-8x you might see if you go all the way to hand-tuned SIMD).