This issue has been around in previous releases too. Pretty easy to reproduce.
- Start a command that requires multiple picks, like Polyline.
- Drag the Command Options Dialog off on the other screen.
- Pick your first point and move your mouse to select the second point.
There should be a significant delay in the display of the preview line.
To compare, do the same thing but have the Command Options Dialog on top of the Rhino window. The performance should be quite noticeable.