Plug.Static, through Cowboy and Ranch, offloads the actual transmission of the data to the OS, using :file.sendfile/5. I imagine it has to do more work initially than your alternative, but depending on the file size and the achievable transmission rate it may actually be more efficient when the transfer takes longer.
It might be interesting to run some more tests with different file sizes and transmission rates (preferably on a real network interface). Maybe the optimal solution is a mix, serving small files from persistent_term while falling back to sendfile/5 for large files.
