async/sync and sequential/parallel are orthogonal concerns. You can write sync code which does work in parallel (on different threads/cores) for something like a numerical algorithm that can be parallelized in some way. Deciding whether something is sync or async is about whether it needs to suspend (in practice, mostly whether it needs to do IO) which is much easier to understand up front. Sometimes it changes of course, in which case you have to do some refactoring. But in a decade of programming professionally in Scala and Rust (both of which have "colored" functions) I can count on one hand the number of times where I had to change something from sync to async and it took more than a few minutes of refactoring to do it.