For some reason first dragstart of Window lags, in my test I use the delivered Actipro WPF Theme Designer to reproduce watch this:
https://www.veed.io/view/7adea46c-e959-4859-b383-67e3464314e0
I'm actually dragging it but it lags, feels like it first tries to "Get focus" and then starts the drag action. Seems like a bug (bit annoying too when you switch between windows and try to drag&drop fast).
You can actually see there is 1-2ms delay when you click on a window to gain focus:
https://www.veed.io/view/bd05422c-f619-4ff0-9113-eb97c920ec65