EMI interference with the bluetooth signal (from USB3).
Try a wired mouse, or a Logitech wireless mouse (which uses its own RF frequency -- not bluetooth).
Another solution, clunky but I'll bet it works:
Get a USB "extension cable" (has a male USB plug on one end and a female port on the other).
Get a 3rd-party "bluetooth dongle".
Put the dongle on the extension cable and "string it out" in the direction of your mouse, let it "hang free".
By locating the bluetooth transmission/receive point away from the USB ports, you should get much better reception.