THE WORLD OF food processors, once you get into it, is fraught with anxiety. And why not? Anything that can chop three pounds of onions in seconds is not to be taken lightly. Then there's the money involved, the power, plus enough corporate intrigue for several novels.

The first question--"Do I really need a food processor?"--is the hardest. Processor manufacturers would love to help you spend your Christmas money. Anybody who reads, let alone cooks, is beset by Madison Avenue's four-color finest--processor ads that whisper promises of transcendent kitchen experiences with the flick of a switch. There are whole cookbooks, even a magazine dedicated to "processor cooking." Naturally the poor slob who turns out meals with only a knife and an old Waring blender feels a little de'classe'.

What do food processors do? A lot. And they do it really, really fast. The biggest and best of them (more later about which these are) will do the following in a matter of seconds: mix and knead enough dough for two large loaves of bread, make mayonnaise, chop several pounds of onions, slice enough potatoes for a gratin for eight, turn raw salmon into velvety mousse, make crumbs out of a loaf of bread. And for all these operations, we are talking about seconds. A minute is an eternity in food processor time.

On the other hand even the largest models can perform small tasks efficiently--chop a handful of parsley, for instance, or grate enough cheese for the top of the gratin.

What won't they do? They don't slice raw meat well, nor are they very good at slicing or chopping soft things like tomatoes. They won't grind coffee. And despite advertising claims to the contrary they are not very good at aerating and therefore aren't the machine of choice for fluffy things such as egg whites or whipping cream.

The major advantages of food processors--their speed and power--are also their major problem. A few extra seconds and your chopped onions are watery mush, or the gluten in your pie crust has been excited to such a degree that the finished crust is tough and shrunken. It's hard to do finely articulated things like slice mushrooms evenly. Processor experts can do it, but it takes concentration and more practice than most cooks are willing to apply.

There are two ways to approach the question of need. One is to ask yourself how often you need to chop three pounds of onions or make a pie crust in seconds. The other approach is to ask yourself if you would do these things much more often if you had a processor. After the first blush of the honeymoon, lots of cooks fall into the I-only-chop-onions-in-mine syndrome, abandoning preliminary fantasies of fine pa te's and nightly rows of perfectly sliced vegetables. Getting to know processors and what they can do for you requires know-how and practice.

But if you've decided that you need a processor, what kind do you need? If you like to cook enough to be able to take advantage of what processors have to offer, you probably need one of the biggest and best. The general consensus of cooks, kitchen store experts and Consumer Reports (see the September 1981 issue) is that the big two, Cuisinart and Robot-Coupe, are leaders in the field. In the last month they have been joined by a brand new KitchenAid processor. Since KitchenAid is known for producing state-of-the-art appliances, especially its huge, businesslike mixer, the K5A, the fact that its new food processor is actually made in France by Robot-Coupe came as a big surprise to kitchen-equipment watchers. Robot-Coupe, as followers of corporate soap opera will remember, also produced Cuisinart processors until Cuisinart began divorce proceedings, taking its name to America and its production to Japan.

Recently I tested three processors. To get a fair comparison with the new KitchenAid (there is only one model, the KFP700), I tested the big Cuisinart DLC 7 Pro and the Robot Coupe 3600. Except for the brand new Cuisinart "X" model, which has a work bowl that holds five quarts and a 1 1/2 horsepower motor, these three have the most powerful motors and biggest work bowls available. All are about equal in power and size. I tried to use them the way a cook would use them in the kitchen. I didn't use any extra attachments, only the blades that come with the machine.

The differences among the three were minimal, but here's a rundown: The Robot-Coupe and the KitchenAid produced uniform bread crumbs faster than the Cuisinart. All three mixed and kneaded bread dough made with seven cups of flour in 60 to 90 seconds. The Cuisinart produced more even, less watery chopped onions than the other two. The Robot-Coupe did the best job on chopping meat for chili, but the other two were close behind. The Robot-Coupe was significantly better at chopping a small amount of parsley than the other two, but the Cuisinart was better at grating carrots. All three were about equal at slicing vegetables, though the Cuisinart's larger feed tube allows a greater quantity to be sliced at one time. The Robot-Coupe and the KitchenAid were slightly better at pure'eing than the Cuisinart.

Cuisinart advertising hits hard on its processor's larger feed tube. It is an advantage when you want to slice or grate large quantities of vegetables, for instance, or make large slices of something like potatoes. But because of the way the "sleeve" over the feed tube is constructed it still isn't possible to drop larger amounts of bread for crumbs or meat to be chopped into the running machine. And the complicated sleeve makes washing up slightly more difficult. I found the Cuisinart two-button on-off system slightly more convenient than the other two because you can operate it with only one finger. (You get spoiled with these machines.)

As of now, the Cuisinart and Robot-Coupe win the attachment sweepstakes. There are no attachments available yet for KitchenAid, but for the others you can buy attachments that make pasta or squeeze juice, and optional blades that will slice fine or thick, make french fries, julienne or grate. With all three machines the metal blade, a dough blade, plus one grating and one slicing blade, are standard.

Prices are also about equal, around $275. But it is safe to say that you should never have to pay full price for a food processor. Kitchen and department stores nearly always have special sales on these machines.