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There is an old story about a king and a wise man. The wise man did the king a favor, and the grateful king insisted on a reward. So the wise man made a modest request: a single grain of rice on the first square of a chessboard, then double it on each square after. The king agreed at once. It sounded like nothing.
By the last square, the king owed more rice than the whole world could grow. Doubling hides the cliff until you are already over it. (Press the squares to feel it.)
Early AI ran into the same wall. The Logic Theorist and the General Problem Solver worked by trying possibilities, following the promising ones, and backing up when stuck. That works while the
But every extra step multiplies the options, just like the rice. A handful of moves is easy. A few more, and no machine can check them all, not in the lifetime of the universe. The thing that made these programs brilliant on a small puzzle was the same thing that buried them on a real one.
That was the real barrier: not delay, but a wall.