Obviously the more data you push the better it is to have a faster interface.
However, in many cases it's a cheap device or slow computer not the USB2.
Consider USB2 bluray players, they work fine playing HD quality video.
USB2 can transfer at 35 MB/s, it depends entirely on the camera and compression on how smooth it looks.
Those cheap webcams aren't exceptional quality.
You have to spend around $80-$100 minimum on a webcam before you get out of the cheap models.
This is why good USB microscopes are expensive.