> I will say that programmers very often have bad instincts for when that's a bad idea

True that. memcpy is basically the literal fastest thing your processor can do, it’s trivially pipelined and can be done asynchronously.

If the alternative is heap storage you’re almost always cooked: that heap space is far less likely to be in L1 cache, allocating it takes time and requires walking a free list, dealing with memory fragmentation, freeing it when dropped, etc.

It’s not a bad short-hand to think of the heap as being 10-100x slower than the stack.