An engineer examines a locust. First the engineer claps his hands, and the locust jumps. He then proceeds to cut one of it's legs, claps his hands, the locust jumps. He cuts one more leg, claps his hands, the locust jumps no more. The engineer writes down his conclusion: when two legs get cut off, the locust loses hearing capacity.
An engineer looks at a high performance ARM core and a high performance, but slower x86 cores. Sees SMT hardware in one, but none in the other. The engineer writes down his conclusion: when SMT2 hardware is missing, the CPU runs faster.
Joke aside, here's another point of view from IBM: