> Memory isn't just slow because they went for capacity
  > not performance (except vacuously), it's slow because
  > of the laws of physics. 

Yes. The farther away RAM is from the CPU code, the more stuff needs to happen before it can get into those precious, precious registers. Even if data from main memory didn't have to travel over a bus/switch/etc between the DIMM and the CPU, it's not physically possible (in any practical sense) to have main memory running at anything close to the speed of the CPU once we're talking about multi-GHZ CPUs. DIMMs and the CPU are running on separate clocks, you have the sheer distance and the speed of electrons through the metal to consider, etc.

  > There's a reason why the iPhone and iPad only have two
  > cores - it's not worth their while adding more but does
  > add lots of cost and complexity.

Yes! There's a reason why the A7 in my iPhone 5S blows away the quad-core ARM chip in my 2012 Nexus 7. That reason is because "adding more dumb cores" is not the answer to anything, aside from marketing goals.