You can use tokio::spawn_blocking to separate compute-heavy stuff, it gets handled in a separate thread pool. You shouldn’t need two tokio runtimes, that sounds problematic.

> How do you properly mix IO/compute (in any language)?

Good question, it’s a really hard problem.

In principle you are meant to use threads, the OS will automatically switch to other work while you are waiting for IO, like with async. But there are difficulties with threads too: memory overhead, switching overhead, thread limits… Although I am getting the feeling that they have a much worse reputation than they deserve. More projects should try switching to threads and actually measuring the difference.