that is because the upstream from the broadband provider is slow. I use my NAS via FTP from all different places. and it works great. Slow to pull something from my NAS because the upstream is only 1.5 mbps, but to send something to it, very fast, as the downstream is 30 mbps. Hassel, bah, took 5 minutes. once set up, it works damn near forever without issue. I have been using FTP to transfer to/from my home network for years now.
this is set up via the router. I would also look into getting a dyndns address set up. dyndns.org for that.