exactly. I can think of at least 5 different projects I have been on where a better allocator would made a world of difference. I can also think of another 5 where it probably would have been a waste of time to even fiddle with.

but as usual there is an xkcd for that. https://xkcd.com/1205/

One project I spent a bunch of time optimizing the write path of I/O. It was just using standard fwrite. But by staging items correctly it was an easy 10x speed win. Those optimizations sometimes stack up and count big. But it also had a few edges on it, so use with care.