Connecting external monitor increases touchpad sensitivity significantly

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When I attach an external monitor to my laptop, my touchoad sensitivity increases, making it more difficult to control the cursor. When I move my finger across the surface of the touchpad over 5 seconds with the monitor attached, it moves about twice as far as without the monitor. When I configure the two displays to mirror each other, it seems the sensitivity reverts to normal. It seems the sensitivity is based on size of the screen. How can I keep my cursor speed from changing when I attach the display?

If it's relevant, I am using a Thinkpad T440p.