[cautionarytale]

Fork/exec time for extra processes is usually unimportant. If data transfer is truly a bottleneck, shared memory is as fast as threading.

These costs, though, are generally trivial compared to the lifecycle costs of dealing with multithreaded code. Isolation in processes greatly enhances debuggability, and it's almost impossible to produce a truly bug-free threaded program. Even a heavily tested threaded program will often break mysteriously when compiled with a different compiler/libraries, or even when seemingly irrelevant code changes are made. It's a tar pit.