Handicapping the Kentucky Derby usually involves many complexities, many judgments which must be made about a number of contenders. But this year a single question holds the key to the race:

What was the significance of last week's Blue Grass Stakes, which Spectacular Bid won by a big margin in dismally slow time?

If you conclude that the time was irrelevant and that Spectacular Bid is in peak condition, you can bet the rent money on him Saturday.

If you believe that the time indicates a marked deterioration in Spectacular Bid's form, bet Flying Paster. Or, possibily, the longshot General Assembly.

Simple as it may sound, the question involves so many subtle aspects of speed handicapping and Derby history that it is virtually unanswerable. At least, it has befuddled the world's greatest handicapper (me).

But I am sure of one thing: Spectacular Bid is by nature the best horse in the Derby field. He may be one of the best thoroughbreds of all time. Just as the time of the Blue Grass stirred doubts about his current condition, the time of his previous victories proved his brilliance.

Last fall, Spectacular Bid clinched the 2-year-old championship by shattering a track record in the Laurel Futurity, running better than either Secretariat or Affirmed had in the same event. This winter in Florida, he ran faster than any 3-year-old of the decade has at the same stage of his career. Spectacular Bid's supposed chief rival in the Derby, Flying Paster, never has run a single race so fast as to suggest he is a great horse.

But there are reasons to believe that Spectacular Bid may have peaked in Florida. Bud Delp trains the horse hard and Ron Franklin rides him hard, whipping him when he is 10 lengths in front, turning what should be easy victories into nearly all-out efforts. Spectacular Bid's final Florida victory was not nearly as fast as the ones that had preceded it, indicating that he might be tired from his winter campaign.

The slow time of Spectacular Bid's next race, the Blue Grass, confirmed the impression the he was tailing off. He covered 1 1/8 miles in 1:50, running even slower than an allowance-class filly did on the same afternoon.

How seriously should a handicapper take a slow time in a prep race? Derby history offers some contradictory answers. In 1974, Honest Pleasure was billed as a superhorse because of his early-season performances in Florida, but even though he kept winning, his time became progressively slower and his Blue Brass was very poor.

The bettors at Churchill Downs still made him an overwhelming favorite, but this times had tipped off the decline in his form and Bold Forbes upset him.

But as often as not, slow times in Derby prep races have been misleading. Affirmed's victories before the Derby were not fast. Secretariat ran poorly in his final prep race. So did Northern Dancer.In these cases, the prep races were no more than tune-ups, and the horses waited until the Derby to show the full extent of their ability.

There is another reason to dismiss Spectacular Bid's Blue Grass time. Sometimes, atmosphere conditions or track conditions or even a faulty electric timer can make the final time of a race flukey. And the time of the Blue Grass does not make sense.

When Spectacular Bid was running his superhorse races in Florida, he was beating Lot O'Gold and Bishop's Choice by eight lengths or so. In the Blue Grass, he beat Lot O'Gold by seven and Bishop's choice by 15. If Spectacular Bid went off form drastically, the other horses did, too. I find it implausible that three horses who seemed to be in sharp condition would all go off form simultaneously.

There may also be empirical as well as analytical reasons for doubting the significance of the Blue Grass time. I asked Delp this week whether it was possible that Spectacular Bid was going off form without his noticing it.

"Off form? There's no way it's possible," the trainer said. "He's perfect. The day after the Blue Grass I couldn't hold him on the ground. He's ready to run the race of his life Saturday." Delp clapped me around the shoulder and led me to the gray horse's stall. "Does that look like a horse who's off form? Does that look like a horse who's tired?" he asked to my layman's eye, it did not.

If Delp is right that Spectacular Bid will run the race of his life Saturday, he will annihilate his competition and could break the track record. But even if the colt is not at his peak, he still can win because of the demerits of his opposition. Flying Paster is not as good as people think, and he never has raced over anything but the rockhard California tracks. The only other horse in the field who has shown flashes of the ability necessary to win the Derby is General Assembly, but he has turned in too many faint-hearted performances to be considered anything more than a promising longshot.

Although he figures to win, Spectacular Bid hardly is an attractive betting proposition at 4-to-5 odds, because of the uncertainties about his current sharpness and the presence of the inept Franklin in the saddle. It is almost un-American to abstain from betting the Derby, but I cannot justify any wager Saturday.I will only be cheering for Spectacular Bid to show the world how great a horse he is. CAPTION: Picture, Kentucky Derby nominee Golden Act is sponged down by groom Mark Oesch after morning workout at Churchill Downs. Golden Act drew first pole position for Saturday's classic. AP