Web Workers are not parallel - they are only concurrent to the main context: think OS threads. Async JS is akin to using very lightweight simulated threads.

You will not necessarily utilize more CPU cores by spawning additional Web Workers because they are not inherenent parallel. The actual performance of Web Workers depend on how your browser and OS schedules threads.

They are OS threads despite the mountain of misinformation on the Internet about them implying that they are truly parallel. They are not.