It appears to me that that the design of some Windows systems is such that using a high speed port is a requirement for the F1.5K. Not quite sure of the technical issues causing this.
Most USB controller chips set their speed at the speed of the lowest speed device that is connected the controller chip. On the PC where I use my FLEX-1500 I connect the 1500 to a USB 3.0 controller chip and I connect the keyboard, mouse, FlexControl Knob (USB 1.1) to the USB 2.0 controller chip. That way my FLEX-1500 runs at full speed (USB 2.0) and it is not slowed down by the other slower devices. That explains why an add-in USB 3.0 adapter yields improved performance. Be sure not to plug any slow devices into your new USB 3.0 adapter.