Here's the thing, it's not just about speed or efficiency, it's also about cost and price. Part of ARM's advantage is that chips are cheap and they have no incentive to support a high margin business that Intel does.

Fundamentally, Intel doesn't want to switch everything over to Atom chips because it would ruin the economics of Intel's business. Right now they would much rather sell a i5 at $200 or an i7 at $300+. Compare that to an Atom chip at say $25 per chip. Intel would need to have 10x volume to make the same money.

So, while Intel wants to be competitive with ARM, the economics of that business just aren't as attractive as the PC/Server chip business.