What about web workers? I believe the whole idea behind them was to allow to run heavy tasks in the background threads. Havent used them personally, but quick search indicates that they are available on node.js via npm [0]. Unless the whole concept is misunderstood by me and they would still be able to block your servers response to http requests.
Yep. Web workers, child processes, whatever works. My preference is to use microservices to keep things isolated. It comes with an overhead of a millisecond or two but for most purposes that's fine.
[0] - https://www.npmjs.com/package/webworker-threads