Because the host system does all the compression and encoding, all DisplayLink devices share the same CPU workload. Thus, the only two variables affecting CPU utilization are resolution and on-screen activity.
Simply moving your mouse around causes CPU utilization to hop up to 15% on a capable mobile Core i5 processor. And that a light workload, as only the screen data around the cursor is getting compressed and encoded.
The absolute worst-case scenario is a contrived synthetic measurement we put together, progressing through Windows' Picture Viewer at the rate of two images per second (though the results of that aren't far off from a more real-world workload of H.264-based video playback). In both cases, the system processes entire frames, causing our Core i5 to hit 50% usage at 1920x1080.
Moving a window on the DisplayLink-enabled screen causes CPU usage to jump to 20-30% for short bursts, depending on the resolution you're using. Once the image stops changing, CPU utilization is allowed to fall back to 2%. That's good news, since we're seeing some scary-high utilization numbers.
Naturally, if you want to avoid those uglier results, avoid video playback on a USB-attached screen. Swap over to the notebook's integrated display and CPU use drops to near-nothing thanks to Nvidia's Quadro 3100M GPU with hardware-accelerated H.264 decoding.
Obviously, gaming on a screen attached via USB is a terrible idea. We wouldn't even recommend watching a movie on it. But as a mechanism for improving productivity via a second or third display, DisplayLink's solution is more compelling.