Yes, he was ruling himself out as unable to answer. So am I. And it would take a *LOT* more than a Google search to answer. I lean towards agreeing with the people who cite bus speed as the limiting factor, but I'm not sure, and there could certainly be special circumstances where something else was the limit.
I *do* know that it's not an easy question to answer, and that any answer is going to depend for its correctness on a presumed workload. (Some things are CPU bound, and don't even use much RAM. Other things are IO bound, and make you think your disks are thrashing. Most things are somewhere in between.)
But the original question was "has the gap between fast-small memory and slow-large memory gotten larger, smaller, or stayed the same. Even that's an oversimplified question, as it doesn't deal with persistent RAM. (I'm dubious about the value of that, but some people used it to advantage in the days of core-memory.) Also ignored were the questions of relative cost. If you pay ENOUGH there are lots of exotic technologies...and I have no idea of the tradeoffs.
So much better to get the answer from somebody who actually knows the area. It's not simple.