Hi Martin, it's good to have your help again.
I've uploaded 3 videos here:
allow-transparency.mp4 shows dragging a RadWindow with RadWindowInteropHelper.AllowTransparency="True", which is quite laggy;
no-transparency.mp4 shows dragging the same window with RadWindowInteropHelper.AllowTransparency="False". The movement of the window seems smoother, however the window content jitters;
edge.mp4 shows dragging an Edge window of the same size for comparison.
Note the videos are taken on a 4K resolution monitor with a DPI scale of 175%. The videos are then downsampled to 1280*720. On my 4K monitor the lag and jitter effects are more visually significant.
Also, I noticed that no matter enabling or disabling RadWindowInteropHelper.AllowTransparency, when dragging the window, the mouse cursor does not stay in place, but shifts towards the bottom-right of the screen. For example, if the mouse cursor is located at (100, 0) from the top-left of the window, after dragging the window for sometime, it gradually shifts to (150, 50).