Async doesn't solve that problem. What you're asking for is some sort of expression in the type system of expected performance, but there's no tech that can do this. Consider that modern NVMe SSDs can do disk reads faster than many calculations over the content of what was just read. A function that changes its algorithm to have worse time complexity won't be marked as async but could break your app, adding a single disk read would require marking as async but probably won't break your app (especially if it's in a non-hot path), adding 10,000 more reads where previously you had one doesn't require changing your async annotation but might break your app, etc.

So the whole sync/async distinction doesn't really make much sense outside the context of single threaded UI toolkits, where indeed you have to read the docs anyway because doing some very CPU intensive work on the UI thread will block it even if no "blocking" APIs are ever invoked (and what is or is not blocking is somewhat arbitrary anyway in many cases).