I like your take, and I propose there is another kind of division: function "coloring". An async "effect" type (e.g. futures, or even just 'async function') sig

I like your take, and I propose there is another kind of division: function "coloring". An async "effect" type (e.g. futures, or even just 'async function') signals to programmers that there is some concurrency stuff going on around the place(s) you use it and that we need to talk about how we're going to handle it. i.e. rather than a performance issue, it's a correctness/safety/semantics issue.