Busy week... But here's what I would do to start...

1) Check the performance in task manager (second tab). See how the graph looks while you're transferring normally and not.
2) If you look at the bottom of the task manager, you should see "Open Resource Monitor". Click on that, switch to the network dropdown and observe to see what apps are running and their utilization.
3) Check the event logs manager (System and application logs). It might show lots of errors which could indicate that there's an app error/corruption.

Beyond that you would likely need to start getting into something like wireshark which is potentially a whole different beast.