Engadget benchmarked the MacBook Air and declared it the slowest machine Apple makes. Most performance complaints have been about the 1.6 GHz CPU, a 20% clock-speed reduction from the 2.0 GHz CPU offered in the lowest-end MacBook. But that’s irrelevant in practice.

Most people, even those technically inclined, don’t understand some fundamentals about modern relative hardware performance.

Two things matter for general-use performance these days:

• RAM capacity
• Hard-drive speed

That’s it. Certainly not CPU clock speed, motherboard chipset, or GPU. They’re all so fast, even the lowest-end versions, that only specialized applications (video encoding, high-end gaming, etc.) are likely to benefit from boosting them.

Here’s something else you might not have known:

• Laptop hard drives are much slower than desktop hard drives, even with the same rotational speed (5400 RPM, 7200 RPM, etc.). If you already have 2 GB of RAM or more, this is why your laptop is slow. Every time.

Install and watch MenuMeters to see for yourself.

And something that we just learned from the benchmarks at this link:

• 1.8-inch hard drives, like those in the MacBook Air and hard-drive-based iPods, are much slower than the 2.5-inch hard drives found in other laptops.

Take a look at the “Disk Test” section at the bottom of that benchmark table. Not good.

Engadget didn’t publish the drive models used by their various MacBook Pro comparison models, but I’d guess that the lower one is 5400 RPM and the higher two are 7200 RPM.

Look at the score on the iMac, using a 7200 RPM, 3.5-inch desktop hard drive. It’s double the performance of the laptop drives.

And look at the score of the MacBook Air’s 1.8-inch hard drive: it’s 40% slower than the good laptop drives and 70% slower than the iMac’s desktop drive.

The less-informed tech journalists are complaining that the CPU’s clock speed is 25% slower than other laptops, but they’re completely missing the point.

You could replace nearly anyone’s faster CPU with a 1.6 GHz Core 2 Duo and they wouldn’t notice. But swap their nice desktop drive with a 1.8-inch iPod hard drive, and the performance will suffer so badly that they’ll come over and kick your dog within minutes.