For a simple task of iterating 100 elements multi-threading the task will not provide a performance benefit. Iterating over 100 billion elements and do processing on each element, then the use of additional CPU's may well help reduce processing time.

When would you not use multithreading?

Actually, multi threading is not scalable and is hard to debug, so it should not be used in any case if you can avoid it. There is few cases where it is mandatory : when performance on a multi CPU matters, or when you deal whith a server that have a lot of clients taking a long time to answer.

Does multithreading affect performance?

Simultaneous multithreading (SMT) improves CPU performance by supporting thread-level parallelism on a single superscalar processor [24]. An SMT processor pretends to be multiple logical processors. ... Thus the performance of an SMT system is intrinsically lower than that of a system with two physical CPUs.