I'd say the Orcs are slow-witted because they were bred for only one purpose: to kill. They don't really need anything else to serve this purpose for their masters. Therefore, I think the "intelligence" displayed by the Orcs in Cirith Ungol was simply a display of their singlemindedness.
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"The hands of a king are the hands of a healer, and so shall the rightful king be known." ~Ioreth~
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