Latency differences aren't just a difference in degree. They're a difference in kind. Some interactions have to be sufficiently fast or they shouldn't happen

Latency differences aren't just a difference in degree. They're a difference in kind. Some interactions have to be sufficiently fast or they shouldn't happen at all. A runtime that minimizes global latency guarantees isn't sufficient. You also need to guarantee that certain individual Actor interactions are sufficiently fast, which is only possible through colocation. You could add a colocation constraint to your Actor scheduler, but now you're pretty much back to where you started. And latency is only one of the problems. Partial failure is another. It's often convenient for authors of colocated actors to assume fate sharing. If they can't, they have to handle a much wider set of failure modes.