[GolDDranks]

I think that data parallelism is a very much wrong tool if you are interacting with external APIs – that's in the realm of concurrency and asynchrony. (Check out the work on Futures-rs and Tokio, they are inspiring projects!)

Rayon requires the callbacks it calls to be `Sync`. That means that they are declared to be safe to run across threads. That means that the API it provides explicitly requires the calculations to be parallelizable (and this is compiler checked), and if it doesn't run them in parallel, that can't hurt, right?