They are real OS threads in all implementations i've seen. As for how heavy, they are definitely a bit heavier than i'd like. But rather than me try to describ

They are real OS threads in all implementations i've seen. As for how heavy, they are definitely a bit heavier than i'd like. But rather than me try to describe it, [1] is a really good benchmark with results that you can run yourself if you want. [1]https://github.com/gmarty/web-workers-benchmark https://github.com/gmarty/web-workers-benchmark