> but so could FFI calls to another language for the CPU bound work

Worker threads can be more convenient than FFI, as you don't need to compile anything, you can reuse the main application's functions, etc.

True! Although in a lot of Node you DO have a compile chain (typescript) you need to account for. There’s a transactional cost there to get these working well, and only sharing the code it needs. These days it’s much smaller than it used to be, though, so worker functions are seeing more use.

I make my comment to note tho that in many envs it’s easier to scale out than account for all the extra complications of multiple processes in a single container.