I know that most of the perceived lag on a modern desktop is not due to the CPU, but inefficient I/O to the hard disk or network

Most perceived lag on a modern desktop comes from excessive abstraction which results in poor coding practices. You could certainly argue that IO bottlenecks or a lack of system resources will certainly have an impact but that impact wont be realized until the environment is somewhat saturated. A simple solution to the hard drive bottleneck is to throw a SATA3 SSD in there instead, or to give a system more RAM to boost disk caching, problem solved. On the other hand, no amount of system resources will alleviate a performance hit caused by shoddy coding. This is the reason that I refuse to use Google docs, the performance is about as good as Wordperfect on Windows 95 because of all the abstraction insanity.

One must only look at the iPad 2 too see that's very possible to make a fast computer with beautiful 60 FPS animations and snappy applications using only an 800 MHZ dual core ARM CPU.

The iPad 2 is about as powerful as my Pentium 4 was back in the early 2000s. Shrinking it down to that level is certainly an accomplishment but it's not worth the shock and awe that you present it to be. It's nice to have a device such as the iPad 2 to fill the time when you wish you had a computer but it is in no way a full desktop substitute.

Ironically, my iPad feels way faster than my Macbook Pro most of the time.

Your MacBook is a fundamentally different device than your iPad. They may feel similar but this is purely superficial, the underlying operations are vastly different. If your MacBook is that sluggish, it's either because you're using an Apple product or you've got a PEBKAC error.

You don't need to be an expert in the microprocessor industry to know that the CPU performance race is over

Yes you do. The CPU performance race has been over for the past 5 years but not for the reason you think it is. The CPU performance race is over because AMD choked and threw in the towel. In 2007 AMD's flagship Phenom processor was bested by Intel's then worst in class Core2Quad Q6600 in almost benchmark (if not every benchmark). In 2011 AMD's flagship octal core Bulldozer processor was beaten by a Intel's worst in class quad core i7 920 from 2 years ago which also had an added handicap of only having 2 of its 3 memory channels loaded with DIMMs. Don't blame AMD's failures on the market, or Intel, blame them on AMD.

The fact that the CPU performance race is over doesn't mean that Intel has won, it merely means that Intel is the only competitor since AMD is effectively now a non-contender. It also doesn't mean that there is room in the desktop market for ARM CPUs, or that desktop hardware manufacturers are suddenly going to start writing drivers for two completely different architectures.

While it is certainly true that ARM is gaining on Intel in the performance space, it is still a long long way behind and that gap is only going to get harder and harder to close as time goes on. This is going to be doubly difficult when ARM manufacturers try to catch up to Intel in the general purpose execution department. It's easy enough to say that ARM has a lead in performance per watt if you ignore all of the special hardware capabilities that Intel CPUs have which are mostly absent on ARM or if you forget that power consumption scales logarithmically with voltage and that voltage is necessary to maintain a higher frequency.

It's all about power consumption now, and X86 fails miserably at lower power computing. Unless you know something I don't.

I do know something you don't. Architectures aren't designed to scale infinitely in both directions on the power scale yet Intel still manages to operate dual core full featured processors in the 17 watt range that will still destroy any dual or quad core ARM processor that gets put up against it. Also, I'm not sure how you can justify your statement "it's all about power consumption" because for 95% of the desktop market heat is a non issue whereas a lack of performance certainly is. If you live in a datacenter the constant whine of fans and AC units can certainly get annoying but as I mentioned above, there are already low power solutions that can be had without reinventing the wheel.