[josh2600]

Djb’s article linked above answers this in detail. In short, vectorized instructions don’t have to hit ram to do things like addition, but they can heat up the processor beyond its ability to cool to operating temperatures. The clock speed drop is necessary to avoid overheating.

For more reading, check out cpu pipelining, as well as how vectorized instructions actually work. The performance benefit for well implemented vectorized instructions overcomes the clock speed hit by leaps and bounds, which is why mobile systems make such heavy use of them, for example.