Not your parent but I think that is exactly the point lots of people here are making.

There definitely are niches where there are quite a few performance optimization opportunities that users do care about.

In your example making something a user is actively waiting for go from 3 seconds to less than one is a great optimization target. What is not a great optimization target is making something the user is actively waiting for and that takes 30ms take 25ms instead. That's wasted money on developer time.

If your "user" is a developer of embedded software with memory constraints and using your library leaves them more room that's awesome. If your user was someone using the library on a general purpose computing device with loads of memory then the 2 vs. 5 does nothing.

You need to do some research on how to get modern C/C++ compilers to vectorize.;-) No assembly required, and not that hard to restructure code. (But MUCH easier in C++).

So far it seems like if you have some parsing or formatting task that can be trivially vectorized, the compiler will never do that, and you absolutely must use intrinsics.

I tried auto-vectorisation, and it worked pretty well. But the code became just as big as using intrinsics would have (that with explicitly unrolling loops and rearranging things in memory), and intrinsics generated code that was easily 35% faster.

I decided not put it off for later, and keep things simple for now.