I recently received a new MacBook Pro laptop for work. I wanted to set it up with dual external monitors in the office. I went to the local electronics store to buy Mini Displayport adapters for VGA. They were out of stock. Not wanting to wait, I settled for USB adapters, instead. *Mistake*! The displays, driven via USB, could not keep up with mouse movement or playing videos. The visuals were choppy, skipping along instead of giving me smooth response. I bought a Mini Display adapter as an experiment.
Testing side by side, I connected one display to USB and one to Mini Displayport. I moved the same video back and forth between the monitors. A huge difference! The Mini Displayport performance was smooth; USB performance was choppy.
I just placed my order for two new Mini Displayport adapters for VGA!