The answer depend on the case. What's the files size ? In most of the cases spawning more FTP client instances will affect the performance, but if you have, let's say 10 files X 5 MB, it's more wiser to fire the transfers simultaneously, thus using the capabilities of the available TCP resources. Otherwise, you'll be sending all those 10 files sequentially, and the total time for transfers will be ~10 times slower. But once again, you need to go more into details, before we can recommend one approach over other.