You can get weird lag issues if the internal display names (the \\.\DISPLAYx) are out of date. Unfortunately they get out of date for various reasons such as plugging in a new monitor, plugging a monitor into a different port on the graphics card, change graphics cards around, driver updates (although this is less frequent currently but does come and go), major Windows updates, reinstalling Windows, switching between surround and not surround, and it probably can go on....