OK, I think I have found something.
The Lenovo Yoga has several configurations; the one I use when attached to the UD-3900 is to have the keyboard flipped under and the display flipped up in a "stand" so I can use it as a second monitor. When you do that, the Yoga turns off the keyboard & touchpad so they don't get accidentally pressed.
Just for kicks, I decided to flip the keyboard back around and try using the touchpad on the laptop to move windows around on the extended monitor. Surprise: the laggy window dragging and rendering is gone. And then when using the wireless mouse lag is still gone. Strange.
I thought maybe it was an incompatibility with the Logitech wireless USB keyboard/mouse itself, so I disconnected. However, I also have a BT Microsoft mouse, and it does the same thing: With laptop keyboard flipped back & disabled, window drags & peek renders lag. As soon as I activate the touchpad, lag is gone, and the BT mouse behaves better.
So, it seems to be some kind of interaction between the automatic disabling of the Yoga's keyboard/touchpad and any external mouse. I will try to see if I can narrow it down with some additional experiments...
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