Very good reasoning by Stephen Hackett on why the switch to ARM processors for the Mac is not that easy. Stephen gives three reasons:

• Performance
• Virtualization
• Product Confusion

On performance:

The iPad Air has a 1.4 GHz A7 chip coupled with a 32.4-watt-hour battery that provides 10 hours of battery life. The 13-inch MacBook Air ships with a 1.4 GHz Intel Core i5 chip and a 54?watt?hour battery that provides 12 hours of battery life.

While the clock speeds are the same, the MacBook Air is computationally more powerful (roughly twice as powerful, according to tools like Geekbench. The iPad Air is a good bit more power efficient than the MacBook, but that difference isn’t a good talking point with such a wide gap in raw performance.