The computer has high APM because it is able to control each unit separately. The actual cheat the computer has on the higher settings is the amount of minerals and gas it receives. It gains resources at a much higher rate then players possibly a 2 to 1 ratio.
It's widely known that the computer automatically has vision of the entire map at all times and on the insane level has some heavy discounts for buying units.
--------------------------------------------------------------------------- Your bitterness, I will dispel
Machines will always be more effecient then players, but they will never be better, so they need to make them challenging and that tends to mean that they cheat.
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