That's the way of getting the address itself, that's unrelated to how fast the actual memory read/write is.
Stack is fast because it is frequently "touched" staying in cache. If you were to continuously read write a small segment of the heap, I don't think it would fair any worse than "the stack". This was my point
Yes, it just adding or subtraction int to stack pointer register. I’m not certain, but the only thing that might be faster is accessing data at a fixed address - that is, global variables.