That only tends to be the case for tiny loops where loop iterator updates can be folded into addressing displacements and that frees up an execution port needed for something else.

If you don't know what you are doing unrolling is just as likely to hurt performance because you don't fit into uOP cache and get less decode bandwidth as a result. Or you increase ICache pressure on macro benchmarks and hurt real world performance. Modern cores are really good at hiding loop accounting overhead.