I would suggest looking around on your switch, firewall, and MODEM for your ISP to see if there is any QoS things setup anywhere.
Make sure your ISP speeds are consistent. Also remember, ISP uses Mbps and where file transfer speeds are based on MB/s. One is data speed, other is file speed. So if you have 30Mbps upload you will only see a 3MB/s download on the file transfer speed.