It depends on what has to wait for the IO to complete. ETW, for example, is a Windows-wide event framework and it would be untenable for everything using it to

It depends on what has to wait for the IO to complete. ETW, for example, is a Windows-wide event framework and it would be untenable for everything using it to wait on the file IO of everything else making use of it. So, it has a solution that makes use of locks but nothing ever has to wait on file IO. File IO is always expensive, the trick is always in how you work around that.