USB-to-HDMI/DVI/DP adapters are not intended to replace PCIe-based GPUs for panoramic gaming. People who need to look into USB video adapters usually need them mostly for 2D such as extra text or otherwise mostly static data/graphics displays. For those applications, USB2 is usually more than adequate.
With 2.5W maximum power on USB2, it isn't really possible to do more than a dumb frame buffer. Even with USB3's high-power profile, you still have less than 5W which still does not allow much of a GPU so USB3 models would most likely be dumb buffers as well....