Wow, you really have no idea do you. Yes, everyone here understands what you're saying, it's pretty damn obvious to anyone who has done a basic algorithms class. Of course he was talking about this particular instance, where it would be significantly faster to use an array.
The instruction lookup code is likely to be in one of the inner loops of the program, so the overhead of calculating the hash function will add up very quickly, compared to simply adding the instruction code to the base address of the array.