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I'd do a test (ideally in the test sources of your project) to compare the time taken by both approaches, using sample data if needed, because it depends on what you are doing, where and on which scale, which we don't know

I would say use multiple if they logically make sense such as downloading two images in parallel. Wrt your overhead question - coroutines are specifically designed for this aspect. It has been emphasised extremely that they are super lightweight threads and thus the cost of switching is negligible

Please move this discussion to #coroutines, where it clearly belongs.