For a long time ago there have been APIs to do asynchronous file I/O on the books for Linux but they weren't worth using because they didn't really speed anything up.
They sped up things for a long time - but only when using unbuffered IO. The new thing with io_uring is that it also accelerates buffered IO. In the initial version it was all through kernel worker threads, but these days several filesystems have better paths for common cases.
