If you need to optimize the allocator you are doing it wrong.
A 0.5% improvement may not be a lot to you, but at hyperscaler scale it's well worth staffing a team to work on it, with the added benefit of having people on hand that can investigate subtle bugs and pathological perf behaviors.
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.
void* malloc(size_t size) {
void *ptr = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_ANON, -1, 0);
return (ptr == MAP_FAILED) ? NULL : ptr;
}
void free(void *ptr) { /* YOLO */ }