You can generally go with the rule of thumb of "one foot per nanosecond" for the speed of light. Signals rattle along data lines a little slower because they are contained on a circuit board and not in free space, so it's more like "150mm per nanosecond" is the metric equivalent of propagation delay. That sounds pretty fast - and it is! It's like 50-60 percent of the speed of light.

But your average 3GHz multi-core CPU is doing a half dozen instructions per nanosecond. That means if you have your RAM over here and your CPU over there on your motherboard, you're going to have wait a measurable, impactful, amount of time for data to go back and forth between the two.

So the mobile/Apple idea of sticking your RAM directly on top of your CPU has some merit.