Asynchronous I/O is only useful if you have something else you can be doing while you wait for file-data to arrive. For example, if you can do calculations on the part of the file you've already read while you wait for the rest to arrive, or if you can process data for one client while data from a different client is in-flight. Databases are a good example - usually they're handling queries for multiple clients at once, and even within a query they can be reading from multiple places simultaneously (like the individual tables involved in a JOIN).

If your program needs to read in an entire JSON blob (for example) before it can do anything, or if it only does light processing on each individual part (like adding up a column of a CSV file) then async I/O probably isn't going to help.