There are two ways to describe the new Compaq Portable III, which is to ordinary personal computers what a Corvette is to a Yugo. You could say that this American-made gem is a nicely designed, well-built PC that also happens to be extremely fast. Or you can say that the Portable III is a screaming speed machine that also happens to be an excellent all-round computer.

The second description is probably the better one, because the most noticeable thing about the new Compaq portable is that the computer was built for speed. It uses an 80286 microprocessor (that is, the IBM PC-AT chip) set at a clock speed of 12 megaHertz. That means the computer's central processing unit completes 12 million operations per second, 50 percent faster than the IBM AT and its clones. The hard disk that Compaq provides has an access time (the average time it takes to find and retrieve one item of data from the disk) faster than almost anybody else's. And Compaq has used six proprietary chips of its own design to speed up processing and data transfer within the computer's circuitry.

The result is a machine that races through ordinary computer operations faster than anything you've ever seen. The main advantage here is for the user who regularly recomputes huge spreadsheets or digs into data bases with thousands of separate records. In that kind of heavy-duty work, the Portable III can make an enormous daily difference.

For those with more modest computing chores at hand, the speed advantage of the Portable III shines through in the acceleration of routine tasks. Programs load and run almost instantly (even that notorious sluggard, Microsoft Word, snaps up on the screen in the blink of an eye). Saving a long text final on the disk takes a second or two at most; spelling checkers and electronic thesauruses seem to search their dictionaries at the speed of light.

As noted before in this column, the actual time saving in all this could easily be described as minimal. On a standard IBM-XT, it might take 10 seconds or so for Microsoft Word to close itself up and disappear after you give it the "quit" command; on the Portable III, the same operation takes a second or two. So what's the big deal? It's hard to imagine anybody who is so busy that saving nine seconds a few times a day is going to make a major difference in productivity.

And yet, those nine seconds can be addictive. After using the Portable III for awhile, it becomes almost impossible to go back to the tortoiselike pace of a standard PC or XT. There's an analogous situation in the world of telephones: A touch-tone telephone probably saves you no more than five seconds every time you dial a number, but once you're accustomed to touch-tone, a standard rotary dial seems impossibly slow.

Even when running at the optional "low" speed of 8 megaHertz -- an alternative provided to assure compatibility with all PC-AT software -- the new Compaq is a wonderful machine to use. It comes with a clear "gas-plasma" screen that provides a bright reddish-orange display almost as sharp as a standard Hercules display screen.

If you hook up an external color monitor, the display will jump back and forth from one monitor to the other at the tap of a key. The color graphics display that comes with the standard unit is fairly disappointing, at least for anyone who has grown used to the IBM Enhanced Graphics Adapter (EGA) standard. You can buy an EGA adapter board for the Portable III, but to install it you also have to buy a separate expansion unit that snaps onto the back of the computer.

As with other Compaq models, the name "Portable" involves some degree of hyperbole. The Portable III is closer to a "luggable;" it weighs 20 pounds (more if you've added the expansion unit). Compaq provides no battery power, evidently realizing that this computer is just too big to use on an airplane or in the car. It is easier to move than a desk-top computer, but only Arnold Schwarzenegger could consider it a portable.

The one flaw with this fine computer is the keyboard. It is hooked to the computer with a short diagonal cable that severely restricts where the keyboard can be placed; there's no way you could hold it in your lap. Further, the keyboard is too slow for a speedy typist and misses a lot of double letters. To its great credit, Compaq is perfectly honest in admitting to these faults and says they will be fixed soon.

As with other Compaq computers, prices are hefty. With one floppy and one 40 megabyte hard disk drive, the Portable III runs about $5,000. With a smaller hard disk, the price drops slightly. Which just goes to show that, even in personal computers, you can't find a Corvette at a Yugo price.