Readynas NV+ slow transfer over internet with windows client

I have had both a Readynas duo and a Readynas NV+ for years. Internally to my own network everything works perfect. Both units are setup to host my personal webpage and ftp. I had a friend try to download a 15mb file from it over the internet and it was at only 50% after 24 hours. After days of troubleshooting what I have found is windows clients performance connecting to the NAS via the internet is pitiful to the point it doesn't even work. Linux, android, Ios and all others do not experience any issues. Thinking maybe Verizon was throttling me I spent an hour with Verizon fios support and found nothing wrong on their end. I setup a computer and installed IIS, created a quick website and had tried that. Windows client over the internet, perfect. I also have a friend with a NV+ configure HTTp on his and he has the same issue. I have tried changing MTU size, change ports, cables etc, no luck. Can anyone tell me why the performance is terrible to these things over the internet when using a windows client ?