Don’t systems normally want fairly tight bounds on when they execute and how long? (Genuine question)

We are considering async systems, but in general I do think that allowing them to suspend execution across frames would be undesirable (at least as a default).

What kind of use case would want to suspend within a frame?

Well, I know I just want to be able to poll futures from a system.

There's a rust FRP library called futures-signals that would be really useful for taking input from a controller and transforming it into high level, meaningful streams. I'd like to react to those streams. I don't think they'd take multiple frames to poll, but even if they did I wouldn't mind too much.

(I'm new to both rust and ECS so if you have a suggestion for how to approach this problem differently, I'd love to hear it)