I knew it! I had a 'gaming' type very comfortably shaped mouse but it used a specific 2.4ghz adapter. Any time I did a large file transfer over USB3.0, any flash drive replicated the issue, the mouse would become jumpy and unusable plus WiFi speeds dropped to maybe 10% for the duration of the transfer. I ended up giving the mouse to a friend and going back to my first gen MX Master.