Pergunta comum em entrevista de Node.js. A resposta abaixo está em inglês, do jeito que dá para falar numa entrevista: leia, feche e tente explicar com as suas palavras.
JavaScript runs on one main thread by default, though Node also uses internal threads. Each cluster.fork() creates a worker process. Your code or process manager chooses how many workers to run. Cluster does not automatically create one per core.
For CPU-heavy JavaScript, a bounded Worker Thread pool can run work in parallel within one process. Some buffers can be shared, which introduces synchronization responsibilities. Processes or a separate job service are other isolation choices.
Check the API first. Some asynchronous native APIs, such as crypto.pbkdf2, already use the libuv worker pool. Wrapping every such call in a Worker can add overhead without helping.
Source: Node.js Worker Threads.
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