Multiple FTP scripts submitted to a subprocess

Why does running 5 FTP scripts sequentially take longer to run (16 seconds)
than when all 5 are submitted to a subprocess and run simultaneously (7 seconds)?
I would think there would be some I/O contention.
There are about 10 files to be FTP'd in each script.
Although each script connects to the same FTP site,
each script writes to a separate directory.
I tried just one FTP script and just changed directories before putting the files,
but it still took about 16 seconds.