While I personally love this answer, I have to admit a basic physical metaphor works. If you remember an answer, it is practically immediate. The further back in your records you have to go to find something, the slower it will be.

We have faster ways of recalling notes today than we did in the past, you might say? Well, yeah. In many respects our ram is faster than registers of early computers, too. That all things have gotten faster doesn't change that things which were faster are still faster. (I'd be delighted to know examples where this radically changed somehow.)