Hello,
We'd need log files to get a clearer picture.
We use the graphics card so it needs to be able to manage all these pixels...
USB 3 is more than comfortably able to run an HD screen, you can from USB2 with our technology. You would not see any lag playing video on that external HD monitor, provided the rest of the system is not on its knees on doing something else, of course.
Have a look at the task manager when you get this lag, please. You may see a bug we've identified and fixed in our upcoming release...