Basically, you want anything that has any kind of IO (network, disk, etc.) or which is very slow to compute to be fully async. If you have a function that returns almost instantly, like computing the hash of something, you can keep that sync.
If you're forced to use a slower sync function (say, something from a library that you can't easily change), then you can use asyncio.to_thread() so that it can run in a separate thread without blocking your main event loop. If you don't do that, then you sort of defeat the whole purpose of the architecture because everything would stop while you're waiting for that slow sync function to return.
If you're forced to use a slower sync function (say, something from a library that you can't easily change), then you can use asyncio.to_thread() so that it can run in a separate thread without blocking your main event loop. If you don't do that, then you sort of defeat the whole purpose of the architecture because everything would stop while you're waiting for that slow sync function to return.