slow USB transfer speeds

What would be a complete list of network/hardware 'factors' that influence transfer speeds when trying to copy data from a server (via a mapped drive/file share), to a USB expansion drive. And can you trust the windows 'time remaining' statistic, or is there a way to calculate your own time remaining statistic to get a  more accurate figure?

We have a user who is connected to our private network outside of the main office via a VPN connection (Cisco AnyConnect), and has a requirement to transfer 50 GB of data from a file server to a USB expansion drive connected to their W10 Laptop device. They are just using standard copy & paste via Windows file explorer, as opposed to using a more advanced transfer app. However, the transfer speed is painfully slow, and therefore quoting upward of 16 hours to complete, which is just not practical.

This seems astonishingly slow (a screenshot of the time remaining showed a transfer speed as low as 26.8 KB/s but there seems to be quite dramatic fluctuation in the speeds), and therefore I wanted to rule out each factor one by one to see if there is a way to speed up the process if possible, or identify and remove the main factor slowing down the transfer in case this is a requirement in future. Any suggestions on likely cause for the slow transfer speed, and ‘alternative options’ most welcome.